As per the quote in your question, "he likes to gravitate to people who are stronger." Who was stronger than Voldemort? Given his personality type, he would have been a prime example to switch sides once it became apparent that Voldemort was going to be the winning side. After all everything indicated Voldemort was going to be victorious until he attacked the Potters.

Before the Potters got killed, Voldemort took over almost the entirety of England. Peter might have been afraid for his future, he wouldn't want to get killed. So he might have turned to Voldemort's side and become a Death Eater.

Given his personality, he was merely trying to play safe.

Like most of the people on this page, I consider that joining the DEs or the OotP is more or less the same from the safety point of view...

But what's more important than safety in human society (according to Maslow's hierarchy - yeah, let's do pyschology)?

Love, esteem, feeling like you're part of the team.

Yes, Peter was part of the Marauders, but they did not really appreciate him, he felt despised by them (and by the rest of the world for that matter); I would say he was disappointed by the OotP side. On the other hand, the DEs were a totally new group, with different codes, different people, and - apparently - the same level of safety... a good opportunity for him to start anew, and to join a more "rewarding" group.

In addition, there are several advantages with being the bad guy:

• People fear you.
• They have to show you respect, or face the consequences, which is not the case if you're a good guy.

As Caligula (who was kind of a bad guy according to history books) used to say (according to the same books): "let them hate me as long as they fear me."

This theory is based on an assumption that Wormatail joined the Death Eaters after Voldemort learned of the prophecy.

If I was Voldemort and I had decided to kill the Potters, but couldn't find them or get to them, I would put all my efforts into people who could lead me to them.

It would have been well known that James Potter's best friends were the Marauders, and of those three the weakest and most susceptible to coercion would be Wormtail.

My theory (no canon evidence) is that Voldemort put pressure, whether positive or negative on Wormtail, which he just couldn't withstand.

I don't believe the character was inherently evil, he was just caught between a rock and a hard place and was forced into "What's best for me" mode.

Sirius and Lupin both had the confidence and the natural inclination to allow themselves to die for their friends. How many of us could truly say that when it came down to the crunch we would not choose ourselves over our friends?

Why did Peter Pettigrew choose Lord Voldemort and not Dumbledore?

Peter didn't chose Tom Riddle, Lord Voldemort chose Wormtail.

A choice assumes he felt he actually had a choice. It could have been that Tom Riddle choose Peter. He would have appealed to his vanity and popularity at a time when most of his friends were going off and having lives. James, Remus and Sirius were great wizards, going off doing great things and here he was, no where near their caliber. He would have left Hogwarts to do far less glamorous tasks than the rest of the Marauders. Imagine his envy?

Peter would have been quite susceptible to Tom Riddle's charm and promises. An affect of many charismatic and manipulative leaders, is not realizing how deep you are until it feels too late to turn back.

Lord Voldemort's pitch was superiority over muggles and half-bloods, an inferior feeling Peter would have been entranced by such a pitch. Once Peter tasted that power, the power Lord Voldemort offered, it's doubtful he'd settle for his old friends.

My conjecture: A thoughtful Lord Voldemort sat wondering who, among those who would stand with Dumbldore, could he pick-off and win over, who would act as a spy, who would they trust? Who was weak enough to control and manipulate?

In Peter Pettigrew he would have found a ready and willing participant, someone looking to feel more important than they were.

He wanted protection and thought the Dark Lord was winning.

Sirius basically says that Pettigrew wanted to be allied with whoever he thought was most powerful, which seems like how he consistently behaved.

“When did I ever sneak around people who were stronger and more powerful than myself? But you, Peter – I’ll never understand why I didn’t see you were the spy from the start. You always liked big friends who’d look after you, didn’t you? It used to be us … me and Remus … and James …”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 19 (The Servant of Lord Voldemort)}}

Pettigrew gravitated to whoever he thought was the most powerful and able to protect him. As a Gryffindor at Hogwarts, the Marauders were likely the best group for him to go to for protection - they were in his house and seemed to be highly regarded among the student body. There was a group of young Death Eaters in the making, but they didn’t seem to offer much protection or status - Snape was one and he was bullied mercilessly by the Marauders. Joining them wouldn’t help Pettigrew much, especially as a Gryffindor who’d have to share a common room and dormitory with the Marauders where he’d be “unprotected” from them. In addition, the Marauders were probably the best at bullying during his time at Hogwarts. Being one of the Marauders would mean he wasn’t on the receiving end of any of their bullying.

Pettigrew had at first joined the Order of the Phoenix - he was in the picture of the original Order members. He hadn’t gone straight to the Dark Lord after the Marauders. He’d originally followed in the footsteps of the other Marauders, joining the same group as them.

“Harry’s heart turned over. His mother and father were beaming up at him, sitting on either side of a small, watery-eyed man whom Harry recognised at once as Wormtail, the one who had betrayed his parents’ whereabouts to Voldemort and so helped to bring about their deaths.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 9 (The Woes of Mrs Weasley)}}

However, Pettigrew wanted protection and safety most of all, and being a member of the Order was very dangerous. Many of the people in that picture died fighting the Dark Lord - among the 22 members, there were six confirmed deaths and one who vanished (not even counting the Longbottoms or the Potters, who died after Pettigrew defected -counting them would make 10 confirmed deaths).

“That’s Edgar Bones … brother of Amelia Bones, they got him and his family, too, he was a great wizard … Sturgis Podmore, blimey, he looks young … Caradoc Dearborn, vanished six months after this, we never found his body … Hagrid, of course, looks exactly the same as ever … Elphias Doge, you’ve met him, I’d forgotten he used to wear that stupid hat … Gideon Prewett, it took five Death Eaters to kill him and his brother Fabian, they fought like heroes … budge along, budge along …”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 9 (The Woes of Mrs Weasley)}}

When Pettigrew decided to join the Dark Lord, it seemed like the Dark Lord’s rise and takeover of the wizarding world was inevitable. He basically says so himself (though he’s begging for his life at the time). It seems pretty clear that the majority of the wizarding world thought that the Dark Lord’s rise was inevitable.

“DON’T LIE!’ bellowed Black. ‘YOU’D BEEN PASSING INFORMATION TO HIM FOR A YEAR BEFORE LILY AND JAMES DIED! YOU WERE HIS SPY!’

‘He – he was taking over everywhere!’ gasped Pettigrew. ‘Wh-what was there to be gained by refusing him?”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 19 (The Servant of Lord Voldemort)}}

No one expected his fall at the Potters’ house - when Pettigrew joined him he was at the height of power. After that fall, Pettigrew made no move to find or rejoin him until being forced out of hiding as a rat when he was revealed to be alive and a traitor. He wasn’t loyal to the Dark Lord, the Marauders, or anybody in particular - he went with whoever he thought could best protect him.

In addition, when the Dark Lord was at the height of his power, he was a very deadly enemy - those who opposed him were killed. Pettigrew, as a Gryffindor and known friend of the Potters, would have found himself in danger simply because of that. Becoming a spy and giving information about the Potters made the Dark Lord see him as an ally and lessened the danger he was in.

J.K. Rowling also mentioned this, though somewhat obliquely, when addressing why Snape wanted to join the Death Eaters.

Nithya: Lily detested mulciber,averyif snape really loved her,why didnt he sacrifice their company for her sake

J.K. Rowling: Well, that is Snape's tragedy. Given his time over again he would not have become a Death Eater, but like many insecure, vulnerable people (like Wormtail) he craved membership of something big and powerful, something impressive.
_{^{Bloomsbury Live Chat, (July 30, 2007)}}

Reading these comments, some have dismissed the power of the Death Eaters compared to the Order of the Phoenix, mostly because of Dumbledore. In one of the books, an older member (Diggle, I think) shows Harry a photograph of the earlier membership, telling him who some were, who died, etc. He also said that they were badly outnumbered, twenty to one, by the DE's. Even if he exaggerated, this was a large, popular group. Encounters weren't duels between Voldemort and Dumbledore, either. The leader's power counts for nothing if he isn't part of a given battle. I'd say it's possible Peter did want to join the winning side. He might even have wanted to bring his friends with him until he realized that was impossible. When he did learn of the danger to James's family, he was faced with a choice. Having learned that Voldemort wasn't just a charismatic revolutionary but deadly even to his own people, he chose his own life.