some people are scared of nice guys.

I still don't quite understand.

First a caveat: I've never really had a long-term romantic relationship, if that's what you're hinting at.

...But I did have friends, some very close, and my experience is that people are attracted to 'nice guys.'

In hindsight, the issue was my painful shyness, insecurity, awkwardness, and other things (and a lack of self awareness) that led to me being afraid of maintaining/nurturing relationships. Perhaps I was perceived as a pushover or quietly intense or something, but I wouldn't say that made folks uncomfortable. Quite the opposite.