Most Helpful Guy
Either he isn't convinced that you're "the one" (or at least, "a" one) for him, OR he really just isn't ready for a serious relationship.
Girls have trouble understanding this because, for most women, getting a relationship is THE GOAL - it's the relationship that they really want. Guys generally don't work that way, especially when they're young (under 30 in modern society). Yes, guys want sex, and they want comfort and closeness, but they don't want the RESPONSIBILITY or the LIMITATIONS and EXPECTATIONS that come with a relationship. They don't want to have to go to your family reunions, or get in a fight if they look at another girl, or have to call you every time they want to stay out late. Despite wanting and enjoying intimacy, their FREEDOM is their top priority.
Eventually, most men mature and are ready to settle down and are prepared to trade their freedom for the benefits of a relationship, but in today's world, that doesn't tend to happen to most men until their early 30s.
Your ex hasn't reached that point yet - he still wants his freedom, but he still enjoys intimacy (not just sex [though that's a big part of it], but also closeness and spending time together), and if he can get that from you with without the responsibility of a relationship, then he's happy. If not, then he's probably going to choose his freedom over a relationship, meaning he'd rather lose you than take on the responsibilities and expectations of a relationship.