A merger is needed for economies of scale (or the current theme for this week "companies that are too large to fail"), and to compete with the now 'world's largest airline', the Deltoids. US needs a large international presence, and UAL needs to avoid another bankruptcy. US has some cash, UAL is running low.
US needs a name change, and UAL needs the Northeast feed and Carribean. Lastly, US needs a labor integration once and for all, and UAL has the stapler.
It's been in the works for a loooooong time now - the only hurdle with a Dem admin in DC is avoiding massive layoffs (which is why both are paring down prior to the merge...) Strap in and turn on the seat belt sign cause it's about to get bumpy!
Cheers.