Because you may have issues with trust and maybe insecure on whether he really likes you back. It's just fear, if you really love him, stay with him, don't fear your feelings :)
I have the same problem with my current boyfriend. We've been going on and off now for about 3years. To me I've always thought of people like a puzzle piece. Like when you're putting a puzzle together and you find two very similar pieces and they look like they fit together and they do for a while. Until you find out that the piece belongs somewhere else. It's the same thing, you want him to fit into your little puzzle piece, but something's just not quite right. No one likes an on/off relationship, it's tiring and stressful and puts everyone in a bad mood. When it's going good keep him, see how long it lasts and relish in the moments of hapiness, but when the pain gets too big, you just have to have a point where you must draw the line and say "no more". It's hard, it really is and sometimes people do change. Give it a break for a while if it isn't going so smooth at the moment and get back together not at a "spur of the moment" kind of incident, but build it up over time, have like a good solid month of no fighting or anything before considering getting back together with him. It's a hard thing to let go, which is why you have to keep fighting for it :).
Maybe you are just a little insecure. The thought of commiting yourself makes you a little nervous so you try to avoid it.