From what I understand, Obama is hesitant because there are several unanswered questions about the evidence, including things like the exact provenance, the circumstances of the alleged attack, and so forth. The last time we acted precipitously on intelligence about WMDs didn't end up going so well.
I don't doubt, also, that Obama would rather not get involved in another Mid-East altercation, though I've always assumed that our response to Syria crossing the line would consist of arming the rebels and/or bombing Syrian chemical weapon supplies. But if it turns out the evidence is solid, Obama will have to act, in order to deter other states from developing and deploying chemical weapons, regardless of whether he himself wants to or not.