How do you answer that?
I'm settling for, "Because there's a civil war in progress and he wants to join in."
Seriously. Two years ago he was all for bombing Assad in support of the rebels. Now he's all for bombing the rebels in support of Assad (though he'd still like to bomb Assad as well.)
No, I don't know what Cameron should do instead. But of all the options open to him bombing is the most facile and futile. We should have learned from our own experience (during the second world war and after) that nothing so strengthens a people's will and enflames their fighting spirit as being bombed.