I don't think it's sad at all. Mia was going down a destructive path and she saw she was going to drag the MC down with her. She loves the MC and wants to see him become a great man. So, she left him $20,000 to better himself, while she goes to work with her aunt so she can become a better person, too. Not to mention, the MC was acting like a complete asshole. He started neglecting her when he started messing around with the mom. I would have been mad if she stayed with him at the end. He didn't deserve her. Mia staying would have been bad, and the ending would have not been good. However, with her leaving, it opens up the possibility of a happy ending. We'll see what happens in Summer with Mia 2, but I expect good things, and a happy end (whether it's with Mia or not).