Mia wants to be a movie star, but she fails auditions every time. Sebastian also wants to open his jazz bar, but nobody doesn't admit his jazz. That' s why they have a sense of frustration.
At a certain party, they have a talk. Since that day, they come to talk about their dreams and have a good relationship.
I think it offers a workable model of how to achieve one’s dream in the entertainment business. It shows us the difficulty with realizing dreams in entertainment business, but at the same time, we can know the awesomeness of dreaming.
In the ending, Mia comes Sebastian's bar with her husband. This scene shows that Mia and Sebastian are going separate ways. Mia becomes a movie star and gets married a man. On the other hand, Sebastian becomes independent and can open his bar. They really loved each other, but had to break up to realize their dreams. At the end, they exchange glances and smile faintly. It means they kept telling themselves that the way that they chose was correct.