What makes a guy decide that "she" is "the ONE" and then actually do something about it?

Maybe it's just me, but I'm a kind, loving, reasonably attractive, competent woman who is stuck in the "endless dating cycle". Since I graduated college I have been in three very serious, very long-term relationships. The first never claimed to want a permanent commitment, but we dated for 5 years. The second claimed that I was "the one" and then changed his mind. The third (and current) also says that I am "the one" but he's completely dragging his heels about taking the next step. So, guys, please tell me, what does it take? What makes a guy commit to a girl?