
The idea that girls don't like "nice guys" and only like a$$hole's or jerks or guys who treat them like sh1t is a faulty assumption, it is not correct.
The problem is often a nice guy's definition of a "nice guy"! He think he is "nice" if he does all these nice things for a girl like open doors for her, compliment her, make or buy her gifts, and never say or do anything mean to her; he can't imagine ever "treating her badly." He says he would always treat her like a princess. The nice guy befriends the girl and is there for her, gives her emotional support (like any friend would), understands her side of any issues she might be having with her boyfriend (like any friend would, but for him, he is of course sympathetic to her and NOT her boyfriend, and he wants to curry favor with her). The guy think doing all this SHOULD earn him the girl, that all this work means he deserves the girls' affection, that he has treated the girl better than her boyfriend, and he gets angry that the girl never wants him.
There are a couple of problems with this. The first is that it was unfair for the girl to say that she wishes her boyfriend could be like him. Sees him as a friend but as also as a male and sees only that he understands her feelings. She wishes that her boyfriend could understand her feelings and her point of view just as well as he can. But a relationship has two people who sometimes have conflicting interests and feelings that can cause conflicts and hurt feelings. It takes time, effort, and communication to work things out. It is never as easy as gaining the understanding of someone who is already totally on your side. Or of someone who is willing to do anything to win you over.
The second issue is, you can't buy someone's affections with gifts and favors. That in effect would be worse than buying friendship; it would almost be making the girl a prostitute; do you see?

But the biggest and most important issue, even if a girl doesn't have a boyfriend, girls want a guy they can respect. No one can respect someone who licks their boots! And that is what it feels like when a "nice guy" is fawning all over them, always letting them have their way, agreeing with everything they say. You see a guy and a girl arguing and thing the guy is a jerk. The girl wants a guy with his own opinions, with drive and ambition, who won't let her walk all over him, a guy with some level of what might be termed that "classic masculinity," you know, showing some strength and protectiveness.
So when you see a guy being a "jerk" or "treating her like sh! t," often what you are seeing is an argument because they are disagreeing with each other, or one has upset the other or hurt their feelings. But fighting is essential to a healthy relationship, and so conflict resolution, and respect, and communication, and you don't have any of those things if you are always letting her have her way, putting her on a pedestal, treating her like a queen. How is that interesting to her? What is there to explore and learn? How can she grow as a person and be exposed to new things? Where is the excitement, the passion? No, this kind of "nice guy" is boring, stagnant, a dead end.
So, if your idea of being a "nice guy" is being a people pleaser, a door mat, a guy with no personal ambitions of your own but to "make a girl happy," then you are right, you will always finish last. You will have no one to please, your only purpose in life will be thwarted, and your life will be meaningless, sad, and alone.
But if you instead set other goals and ambitions for yourself, have a purpose, look within yourself to find happiness, then you will become a person who is attractive to the opposite sex without even having to try to win them over. You will be able to meet a girl half-way, each respecting the other, each deserving of respect, each having self respect and self esteem and standing up for your own boundaries and feelings. The only downside is that you will then be considered an a$$h0le by other so-called "nice guys," the type of guy you used to be.
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