Attacking Libya was the Europeans' idea—not just for moral but for self-interested reasons—and we're going along to pay them back for helping us in Afghanistan.
When Clinton tried her humanitarian shtick on Face the Nation, Bob Schieffer pointed out that we aren't attacking Syria's ruling family, which has massacred many thousands of dissidents. To this, Clinton could only answer, "Well, if there were a coalition of the international community, if there were the passage of a Security Council resolution, if there were a call by the Arab League, if there was a condemnation that was universal …"
In other words, morals alone won't move us to attack. We'll do it only if other nations care.
There's really not a good reason for us to be in Libya and not in Syria or Yemen.