Humanitarian reasons? There's a dictator killing his people off. It would be good to have allies in that part of world for a change. Those people deserve better than Assad.
Our "Creating allies by freeing them from dictators" plan hasnt gone as well as we hoped in the past. Why would it be different this time?
If we were justified by going to war with every single nation where a dictator is killing off his people, we'd be in most nations in the world right now. What other justification would we need?
What exactly do we gain by this? If we dont gain anything, i don't see why we should get involved.
I'm not saying we should get invovled, and your points are valid, but the reasons we woul get involved would be to stop Assad and maybe pave the way for a democratic free ally in Syria if all goes. It's a long shot I know. Libya did not cost much in terms of money or lives, so maybe an opperation like that would serve us well, I don't know.