I did this. I was at uni and a guy on my course who I got to know and hit it off with asked me out several times. We got on very well but I just couldn't date him. I felt so inferior to him. He was a perfect gentleman, kind, funny, laid back, confident and very intelligent and good looking too. He was never short of female admirers.
I felt he was too good for me, way out of my league and didn't understand what on earth he saw in me. The fact that he was even my friend felt weird to me. I actually really liked him a lot and cared about him but I think because I did care about him, it made me want to kind of protect him from me if that makes sense.
I never really thought about this until recently and felt quite saddened really. Why would I do such a thing.