Why do so many guys have problems committing?

Okay before I start and to try and minimise the accusations of sexism, even though, yes I suppose this is slightly sexist, I would just like to state that I know there are plenty of guys who want fulfilling relationships and have no problem committing to one woman. And I know that there are also plenty of females, myself included, who also have commitment issues but in my personal experience you more often than not hear women complaining of men not wanting to commit and men complaining about women who are too smothering.

So apart from that the question pretty much speaks for itself, however I don't just mean having problems committing to a relationship, I mean problems also committing to the opposite: a breakup. 

I would be the first to admit that I have commitment and intimacy issues. I enjoy casual dating but I never seem to be able to bring myself to take something further. 

However I always insure that when I end things with someone it's ended completed. No take backs. No yo-yoing back and forth. If you're out of my life you're out of my life. It's not fair to keep someone dangling on my hook once I've made the decision to cut the line. 

But I find this isn't the case for most commitment phobic males. Not only can they seem not to be able to commit to a relationship but they also seem to not be able to commit to not being in your life. The number of times I've had a boy tell me it's over just to come back a few months after is astonishing. It makes no sense to me and it's just plain cruel. Why are they so indecisive?