my father was always fond of saying that boys need to sow their wild oats before they would be able to settle down happily. i know there are some exceptions to this rule, i've met a few that were ready to be loyal husbands since they were little boys, but i think it probably holds true for the general majority.
if your boyfriend is young-ish (under 30), especially if he hasn't had much 'oat sowing' time prior to dating you, he may be feeling that itch. i don't think its really right or wrong, probably just a vestigial instinct telling them to populate the world. he may be a great guy, may even be a great guy for you... but he may not be ready for a long haul commitment right now. even if he wants to be, even if he thinks he is... usually that chaffed feeling that leads to fights when there's no reason for one, or the longing gaze at the greener grass are brought on by that impulse.
maybe try taking a break. dating other people. you may find what you were lacking with someone else, or you may come back to eachother a little wiser and a little more ready.
good luck