Did they explain how she ended up with that loser?

Michael Angarano and Juno Temple, the actors who play John and Alice, are actually dating and real life:

http://www.entertainmentwise.com/news/109280/Kristen-Stewarts-Ex-Michael-Angarano-Calls-New-Girlfriend-Juno-Temple-The-Best-Young-Actress-Around

it's left to the imagination but honestly i think you could get a little creative ad imagine your own back story...
it's mentioned they met in high school, so there's that.
and then john obviously charmed alice somehow and voila. he's a funny guy and other than the fact that she's gorgeous was she really that much of a catch? she didn't have a job either..

and thats awesome that they date in real life. yay :3

When she said "I'm too good for you", I honestly thought, knowing everything I knew about the 2 of them at that point, that she actually had it backwards. She was irritating and self-righteous, and he was a nice guy always trying to do the right thing, but coming up short. I guess it depends on your point of view.

Well, she's also a loser, so....

They were together because they were both sweet (not to mention attractive) people. It hurt them both to say mean things to one another and in the end, they gave up on all their hurtful actions, giving up large amounts of money because in the end, they valued each other more than any amount of cash. Plus, as Dr. Ling says at the end of the movie, they had the strength of character to do the right thing with the teapot, something others over many hundreds of years were not able to do. Plus, they were both the type of people who would never refer to a fellow human being as a "loser". ;-)

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Spoiler. Yes she was even more of a loser. He was the honest one because he tried to return the teapot that she stole to the woman at the antique shop. Unfortunately for them the shop was closed.