Sometimes it really has absolutely nothing to do with the fact that you're a nice guy. Sometimes, believe it or not, it is not your generous nature that puts me off, or that I have daddy issues. No, every now and then, I just don't feel the connection.
I'm not throwing all "nice guys" into the same category, but in my personal experience, I've seen nice guys do everything by the book, and walk away dejected and rejected, wondering what they did wrong. What they don't seem to realize is that it isn't always what they did wrong, but rather what was wrong about what they did right.
Often enough, I see these guys do all the right things, but at the wrong time. You put all your romantic efforts into wooing the lady, doing everything you can to win her over, not even stopping to check the awkward signals of rejection along the way. I have nothing against nice dates and romantic gestures, in a relationship. But nothing makes me more uncomfortable than receiving all of that while still getting to know a man.
I have had quite a few friends come to me over the years, frustrated that the more they protested gifts and attention, the more they tried.
The problem seems to be that these guys didn't understand that they were rejecting the offered attention and gifts because they didn't know where it was going, or where they wanted it to go, and did not want to feel guilted into it after letting them do all those wonderful things.
Yet they insisted, they felt increasingly uncomfortable, until they finally rejected him altogether, upset he had not gotten the message in the first place. He responds by being upset they had not rejected him, over the attention, in the first place, instead allowing him to continue to pursue them.
The reason they don't reject you off the bat is because in the beginning you do actually have a chance. It's just either the wrong time and they really want to wait, or they don't know you well enough. Treating them as if they were your girlfriend is genuinely sweet, but if they are protesting and feel uncomfortable, you need to recognize that it's not a sign that they want to be treated badly, or want a "bad boy" over a "nice guy". It simply means it's not the right time.