You're right... One's actions would show that they are a nice guy..
Other people have to declare you a nice guy otherwise the title is by self appointment and who the hell need that?
Oh yes you are!!
I think your right
I suppose the problem is that most people think they're "nice". Apart from those few arseholes they always manage to dredge up for reality shows, not many people would admit to themselves that they're selfish or unpleasant people... if they were aware of it, they'd probably try to change it (and even people in this category would probably class themselves as "good people" just by virtue of their vague intention to improve themselves).
A nice guy wouldn't say that he's a nice guy
You are right! I'ld never tell someone I'm a nice guy!
If anybody tells you that they are a "nice guy" you can be sure that they are trying to scam you.