Why did Sheev Palpatine turn to the dark side of the Force?

Sheev Palpatine's early life is slightly explored in the novel Darth Plageuis. We don't get an awful lot of camera time with him, but we can ascertain that he has a number of key drivers.

He has an overweening sense of individual superiority to those around him.

“Some of us are required to do what others cannot,” Plagueis added in a conspiratorial way.

Without a word, Palpatine nodded.

He has a huge and untapped wellspring of anger, initially focused on his father, but later against all of those he views as weak.

Palpatine’s fury buffeted Plagueis. Blossoms growing along the sides of the pathway folded in on themselves, and their pollinators began to buzz in agitation. FourDee reacted, as well, wobbling on its feet, as if in the grip of a powerful electromagnet. Had this human truly been born of flesh-and-blood parents? Plagueis asked himself. When, in fact, he seemed sprung from nature itself. Was the Force so strong in him that it had concealed itself?

Years of repressing his Force powers have driven him to degeneracy.

Bottled up for seventeen standard years, his innate power had finally burst forth and could never again be stoppered. All the years of repression, guiltless crimes, raw emotion bubbling forth, toxic to any who dared touch or taste it.

He's a (pro-human) racist, largely as a result of having grown up on a planet where non-humans have been instrumental in blocking their progress with the greater galaxy.

“Exactly. Humans deserve to have the upper hand here.”

Plagueis could not restrain a smile. “There are many worlds in the galaxy where the matter of who has the upper hand, as it were, is in dispute.”

“That’s because most beings are afraid to take charge. Think what the Republic Senate might accomplish under the leadership of a strong being.”

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We also learn a little about the things that impact on his behaviour after he gains power.

Born into a minor royal family, he doesn't care about money.

Palpatine shook his head. “Credits don’t interest me.”

He's not fussed about fame or expectations.

“The Naboo might expect more of the eldest son of House Palpatine.”

“I ignore the expectations of others,” Palpatine said without looking at him.

He's something of a brat (and a moral relativist to boot).

Here was a youth who had already risen above common morality and had judged himself unique enough to create an individual code of ethics.

He thinks that peace = stagnancy / He thinks that the Jedi are weak (see above)

Palpatine shook his head. “I don’t view it like that. I think that the Jedi have dedicated themselves to limiting change. They wait for the Senate to tell them when and where to intervene, and what to fix, when in fact they could use the Force to impose their will on the entire galaxy, if they wanted. I’d have more respect for them if they did.”

He's very interested in immortality.

Seeing through him, Palpatine snorted. “Just how old are you?”

“In human years I would be well over one hundred.”

Palpatine whistled. “I envy you that.”