I love this conclusion. They haven't known each other very long, but the way the respond to each other is undeniable. And I love that Aria's sister is the one who makes her see what she has to do.
Aria is hurt, and doesn't want to fuel the pain her past can cause Sebastian. But she isn't happy without him. We learn the awful details of what has Aria pushing away men in powerful positions. Sebastian learns what he did to make her want to walk away, and feels awful for it.
This conclusion is great for these two characters, and there's hope that we will see them pop up again in Ethan and Chloe's story.