Why exactly is it haram to have a girlfriend?

This practice is haram based on the Quran/narrations -- and is forbidden naturally due to its negative effects on both friends (individual), and on the society, etc.

From Quran/narrations's viewpoint: (I mention some verses/traditions):

Whoever jokes with a non-mahram woman will be imprisoned in the fire of Hell for a thousand years for every word he says to her.

Whoever shakes hands with a non-mahram woman will be chained around the neck on the Day of Judgment, then he will be ordered to be set on fire.

Imam Ali (a.s.): O servants of God! Know that ... talking and mixing men with non-mahram women will cause calamity and misery, and will distract the hearts, and constantly looking to the eyes of women, and also look at the non-mahram with the corner of the eye. Doing is the trick of the devil.

This friendship can become a trap for you, and we have no right to destroy ourselves with our own hands. «لا تُلْقُوا بِأَیْدِیكُمْ إِلَى التَّهْلُكَةِ» (بقره (2)، آیه 195

Among the other reasons, which presumably have made its forbidden, amongst:

Psychologically and socially destructive effects of friendship with non-mahrams

Weakening of the nerves due to being single (unmarried); since majority of such haram relationship are transient enjoyment, and their mental/material individual or collective negative effects are much more than its short enjoyment ...

P.S. I myself have a personal related-example/notion for it too, as follows: such friendships will often finished, and will be led in making them to delay/stop getting married, which have diverse destructive results in them, their families, society, etc. Besides, a man/woman who don't control his or her eye/behavior with non-mahrams, won't have much enjoyment from the one who is going to get married with; since he/she had already enjoyed with others ... and his/her husband or wife will be much usual for him/her, that will be led in a bad life or eventually in divorce; and factually such short enjoyment will bed led in missing a more factual enjoyment... Finally, let's not say that: I differ with other! as presumably everyone wrongly says so!

Reference:

https://article.tebyan.net/239552/

https://www.mehrnews.com/news/2155476/

It is widely agreed upon in Muslim tradition, that if an observant Muslim couple is interested in getting acquainted for the sake of marriage they may do so, but in the company of others or in public places.

For Muslims, the idea behind this is that having a chaperone prevents the couple from acting in ways that they would not be likely to in the presence of others. (Source: Can Muslims have boyfriends/girlfriends or date?

Hence, there is no harm in trying to know her, but Islamic conditions must be met - there must be someone around who is a mahram and can understand and explain the reality. So if it is to understand each other because you want to get married, then by all means within the limits of the shariah.

https://www.islamiqate.com

Having Girlfriend, is undoubtedly haram is Islam. But however your question is, then how can i know the girl that i would like to marry?

To answer this. There is halal ways. If you like a girl and want to marry her, tell your parent about it and your parents would talk to her parents. Don't have relation with that girl privately. Talk to her in front of your parents or her parents. By that way you can get to know her in Islamic way. if you're going to message her on whats app, create a group and let the girl add her brother in that group.

The Prophet ﷺ Said :

A man is not alone with a woman but the third of them is Ash-Shaitan.

al-Tirmidhi, 2165

Also Watch this VIDEO for detailed explanation.

When you confuse about halal vs haram, think like this...

From an Islamic point of view, every action you do should have a Niyyat and a purpose. And your comfort is not considered as purpose.

I think it is applicable to almost anything. Including music.

That means, an Islamic way of life ( Deenul Islam ) emphasizes prior knowledge of what are you going to do, why are you doing it and how are you going to do it. Doing stuff on the flow is not the way of Islam.

Prophet said: " Deeds are [a result] only of the intentions [of the actor], and an individual is [rewarded] only according to that which he intends."

when it comes to your question, There are few things odd in it.

Definitely it's not Haram to have a relationship. Unless it's kept a secret and there is no formal contract between the spouses. It's not OK to meet somebody on one day, mingle a little and sleep together and split the day after that. Every relationship is given high importance in Islam. whether it be relationship btw brothers, sisters, parents, children, neighbor, friends, teachers, students or partners. Every relationship is important. The relationship btw Spouses are even more important.

Never get into a relationship, if you doubt it might not work out.

and to the second point Purpose, does having a girlfriend have any other purpose than your comfort?

Getting to know her better. Do you really believe a lifetime is enough to understand another human being. It's a lame argument. Think about how many relationships fail after a long history of knowing each other or dating.

The important fact that Islam proposes is that be aware of your contract, aware of your limits, respect the contracts, respect each other.

And live a life for the sake of your lord. For the after life. Not this life.

And think about it, There is nothing in a person that going to stay the same for a life time, whether it be beauty, shape, sound, wealth or attitude. All of which going to change over time. Even you are going to change over time. It won't be the same person you know today 2 or 3 years later.

Love your spouse because he/she is your spouse.

There is no reason to searching reasons to love somebody.

Sharia sometimes takes precautions. Quran does not say don't do Zina (unlawful sexual intercourse) , it says :

"And do not come near to unlawful sexual intercourse. Indeed, it is ever an immorality and is evil as a way." http://tanzil.net/#trans/en.sahih/17:32

So Sharia makes haram not only the intercourse , but also touching , speaking (in a flirting/romantic manner) , looking with desire . being alone together .. etc.

"So do not speak too softly, lest the sick at heart lusts after you, but speak in an appropriate manner. " http://tanzil.net/#trans/en.itani/33:32