His motive isn't really all that hidden: he's hoping to get the approval so that when, not if, our involvement gets Americans killed, he can blame them for giving him permission.
^^^This.
Barry takes no accountability for ANY decision he makes, unless it happens to be a good one in the eyes of the beholder.
For decisions such as attacking Syria in which there's an entire shitload of stuff ready to go wrong, Barry will blame Congress should they be stupid enough to go down Barry's Highway.