Another possibility is NRE. I've seen a few relationships break up because of NRE. The new partner is shiny, fun, interesting, because it's new! You are still getting to know each other and there's that excitement there. That feeling that, wow! I've met someone that GETS me! You don't see the faults or bumps in the road yet because it's new. So you start comparing. The more established relationship has lost some of it's shiny so you start thinking something might be wrong with it and really so little is wrong with the new one! (mostly because you haven't had time to hit those bumps or problems that take more communication and work)
It's hard to deal with NRE for the first time and you'll find lots of threads here on it. My advice is a bit different. Try and take those new happy fuzzy feelings and bring them to the established relationship. Remember when you felt that excited and giddy with him? Don't assume that the new shiny is better and just dump the established because it's a little more work to get to the giddy feelings.
I only say this because I HAVE seen people that decide the new relationship must be THE relationship for them and then after they've lost the established relationship and the new shiny wears off, they wonder what they've done. If there's issues in your established relationship, by all means, work them out! Figure out what you don't like, feel is lacking, and communicate that so you two can both work on it. If he doesn't want to or sees no reason to, yep totally bad sign. However, he may have no idea right now that you are having second thoughts about things. Especially if they were never seriously discussed as a problem before.
R New shiny is exciting! Established not working, then work on it. No one reads minds, so can't tell if you have problems if you don't say so. Giving up a relationship because of unmet needs makes sense, giving up one because you don't voice them, a bad cycle to get into.