She Makes it Awesome
Puck is not the kind of guy who's into doing nice things from girls unless it benefits him. With Santana, it was always about sex and nothing more; they were both lonely and used each other to make them forget it. Rachel was a horrible experiment; he wanted to make his mom proud as much as the next guy but nothing was worth having to listen to Rachel going on about, well, herself. He has honestly liked Quinn but if he was being honest with himself, he had wanted her because Finn had her and he was jealous. As much as he loved his little girl, he always felt that there was something missing.
He knows now that in all of those relationships he never felt comfortable, never like himself. All his life, he's always played the part of the player and there has never been a girl who has made him willing to give that up. That is, until he started dating Mercedes; up until then, he had never felt the need to actually get to know a girl for who she is and not just what she could do for him.
Puck and Mercedes had started officially dating a few days after they kissed the day that Mercedes had been slushied. It's only been two weeks but, Puck is the happiest that he's ever been. She's the only girl that has really pushed him to go for his true potential, but she does it in a way that's not looking down on him. He loves finding out the little things about her that no one else knows, like how she still sleeps with a stuffed animal or how she watches The Princess Diaries whenever she wants to feel better. He loves that she'll play Super Mario Brothers with him because she knows how much he loves it.
Even though they haven't had sex yet, he's kind of ok with it because he just likes being around her (even though he can't wait to find out more about her curves). He kind of respects the fact that she won't give anything up to him so easily; he's always admired her confidence and he now he can add it to the now growing list of things that he loves about her.
Puck has never been the kind of guy to be totally whipped by a girl (in a good, non dirty way) but he's starting to realize that being with Mercedes makes it awesome.