we all have our own preferences right? I'm just saying you never hear bad guys being all "i don't understand why a girl would date a nice guy. he seems so lame and unattractive to me." (or something to that extent) it's like the bad/cool guys are too busy being them to care but nice guys all so worried about what other boys are doing. I don't understand.
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If by "bad" you mean they treat you badly? If so, It's because the "nice" guys actually care about you and don't want to see you hurt. Women in abusive relationships often become so emotionally battered that they cannot leave the person willingly. This is co-dependence on the part of the abused.
Though if by "nice" you mean guys that refuse to make decisions and cater to the every whim of a girl they are with/going after, then it's because they are trying to manipulate you into liking/dating them instead. This is co-dependence on the part of the manipulator/abuser. It's not healthy; it's not being "nice" either because all it does is build resentments for the future, and often times their dishonesty will show through in other ways.
If by "bad" guys, you mean guys that are straight-forward, honest, self-expressing and independent. Then it's, again, because the "nice" guys are jealous, and they want to manipulate you. If someone is really honest with you, they often times will tell you what you don't want to hear. This can make you sad, and therefor you may turn around and call him a "bad guy".
If by "nice" guys, you mean guys that are chivalrous, kind-hearted, sweet, and kind, but also independent, honest, and self-expressing; then they aren't likely to knock the way another cat swings. Mostly just because it's tacky, but also because they get that other guys need play too, and that sometimes girls aren't looking for a knight in shining armor (LTR-material).
Here is the crazy part: Oftentimes, the first example of "nice" and the first example of "bad" are in fact two sides of the same personality. One is the conservative cautious side, the other is the side that comes out when he is comfortable. This is where that whole dynamic where the girl is like "he's a totally different person than the one I fell in love with". He's the same person, he's just acting like himself (an a**hole) now that he is confident you won't leave him.
The latter two are often (though not always) two sides of the same person as well. As healthy relationships tend to progress, that knight in shining armor may get lazy about some stuff. He may be more blunt about his opinions, he could even *gasp* pass wind in front of you. The chivalry and kind-heartedness aren't gone, they are just underutilized because of comfort. Much like how a girl will let herself go a little (trust me, we don't care) after dating a guy. It's not a big deal, it can be fixed with some simple communication (I mean the chivalry part, It's a bad idea to ask your Girlfriend/wife to lose weight because she let herself go).
How can you tell the difference? It's hard to say really. Honesty and communication are key. Not "good" or "bad" guys. Those are arbitrary labels that can vary even more than what I've put down here. But basically...
It's not about "good" or "bad". It's about honesty.1