Sometimes it's absolutely mutual. I once went on a date (hooked up on POF) and the minute he showed up I think we both realized we were a mismatch. That was only confirmed as the date unfolded.
I didn't really feel like being the one to say it, but later the next day, he e-mailed me: "I don't think you and I are a match, from my perspective anyways, and as you know, the reasons for these things can't be explained. One of things I have always disliked about dating is the fact that we will be hurt and we will also probably disappoint or hurt others along the way. I wish you the best of luck, and I want you to know that I think you are terrific and someone will be very fortunate to find you. Your Friend, Garry. "
The funny thing is, he didn't realize I totally didn't dig him or maybe he did. Either way, I was glad he shut the door. In some cases I'd rather receive the "Dear John" letter than send it. It actually hurt less than me sending it.
-------------------- The sweetest thing you'll ever see in the whole wide world is a happy girl.