I have had plenty of nice guy moments. My attitude changed at my worst "nice guy" moment.
There was this girl that I really liked. I asked her out in the most cringeworthy manner and she rejected me and asked me if I still wanted to be friends.
We were pretty good as friends. She would tell me everything about herself and I would listen and advise her patiently. She would do the same for me in my moments of weakness. Naturally, she didn't want to lose that and neither did I.
But I realized that I wanted something completely different from her. I realized that I was trying to manipulate her into liking me. Even if it succeeded, I would end up making another person miserable.
I didn't have a shred of self respect within me. Why on earth did I think that it would work as first being a friend? Life is not a romantic flick!
After gathering my composure, I told her that I can no longer be friends with her. I told her that I was strongly attracted to her and I wouldn't be honest to myself if I pretended to be her friend again. She tried convincing me otherwise, but it was pretty clear that she just didn't want to be alone herself. But by then, I made up my mind. I couldn't do this anymore. It was time for me to take care of myself for a change.
It was almost a year before she tried contacting me again. By then, I was a freshman in college and my life was drastically different. By then I realized that I wasn't attracted to her. It was the fact that she was one of the few people who actually paid any attention to me. Now, I was well past that phase. I was no longer the semi morbidly obese neck bearded idiot any more.
I pick it up reluctantly. She tells me that she made a mistake and she wants me. I scoff at that notion and did what she should have done the first time she rejected me. I cut the call and block all forms of communication with her. It seems cold, but sometimes that's exactly what we need to wake us up. I do hope that she is doing well in life.