When she stops Sadie at the airport in S3 and Sadie asks her why she's doing this and Marissa says, "Because you make Ryan happy." Marissa loved Ryan so much and all she wanted was his happiness -- a happiness that at that point in her life she didn't feel she deserved. It broke my heart because she did deserve happiness and she'd lost faith in herself, but this scene captured what a beautiful person she truly was inside despite any of her flaws or misgivings. And put in that perspective, her flaws served to make her that much more interesting and beautiful.