Why can't I give the perfect nice guy a chance?
I don't understand. What makes you unable to give him a chance?
I just can't bear that he's such a nice guy! It's so annoying.
You should not give up on someone just because they are too nice. Being nice is a great quality in a person and it sounds like he would make a great boyfriend. 
Don't be so close minded.
But maybe we'd be better as friends.
You might be better as friends, but you'll never know if you don't give him a chance first. If you like him, then I think you should let him know.