Here's that question

I want to point out that usually when a girl says something to this effect she doesn't actually mean you are too nice. Everyone likes nice people. It means she thinks you are a pussy. It doesn't have anything to do with good boy/bad boy, it's just that women want a man who is assertive and in charge, not a pushover. That is my guess. It is hard to respect someone or feel passionate about someone who is a doormat...

Welp, I guess it falls to me to be the voice of dissent here. Whatever the real reason your GF cheated on you, it is definitely possible to be "too nice." As a matter of fact, I have a person from my past that I've nicknamed "Too Nice Brad." He was probably a great guy, but he was so cloyingly sweet and kind and considerate, with no ameliorating edge of sarcasm, or snarkiness, or grouchiness. Nothing to show me that he was in fact human and not a robot programmed to Nice Human. ..
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That's not to excuse your girlfriend's behavior -- she should've probably broken up with you for being "too nice" rather than cheating on you for being "too nice," but that's her issue. It may or may not have been an excuse; I can't speak to that. But yes, I think it quite possible to be too nice, just as it is possible to be too mean. People need balance. No one wants an emotional extremist...

I know I am not a female but here is my two cents from my talking to girls and personal experience. I am taking this partly from a great video http://www.youtube.com/watch?v=ZrnK-qPARYI about "Nice Guys". ..
Being nice is a great thing, I am the nice guy, but at the same time i am nice i don't care, its just natural. What you can't fall into doing is being nice to everyone, and feeling special around this special girl,..but not showing it, just being nice is not enough. Its not just how you feel, its how you appear to feel too, so maybe you have to work on showing that she is more than just someone you are nice too. make the special someone extra special, and tell them that, otherwise they will think they are just another person you are nice to...
sry i realize this probably doesn't help to much with this situation, but yes you can be too nice by just being nice. Act and appear different with her so she knows...

Alright, there is such a..thing as a guy being "too nice",.. If a guy never has a bad day, never gets angry with you, doesn't understand sarcasm and such then he is too nice for me...However, your girlfriend shoudl have broken up with you before resorting to cheating because now your nice-ness just seems like a far grasp of an explanation for why she cheated...But just because she thought you were too nice, does not mean other girls won't love that, becasue they will...You just need to find less of a bitch...

Okay, I've been on the opposite side of things. ..
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Me and my bf dated for a year and I loved him dearly (still do, but anyhow), and he was a NICE GUY. Kind, thoughtful, a gentleman and all whole host of other winning attributes. It was was the last one that gave me problems. ..
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He was too much of a gentleman. ..
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I'd constantly try to convince him that I was a WOMAN, and not a perfect virginal princess and that I preferred to be treated like the former and didn't mind being told that I was hot, gorgeous succubus and that I wanted him to actually be the man in bed, instead of me having to drag him through the act. (Damn you feminism! Scaring our men too much to be manly men!)..
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However, it didn't work. :/ So it was quite hard to maintain passionate feelings for him, especially after I met someone else who was more than willing to treat me like a woman, and I'm very ashamed to say I cheated on my boyfriend and we broke up in the fiery aftermath that was him finding out about it. * sad sigh *..