Jane T answers:
First of all, at the end of the Graduate, Benjamin and Elaine act in the heat of the moment and flee the wedding. When they finally get on the bus and get settled, they both realize that what they're doing is insane, and that they're not heading towards a fairytale, but an unknown and impractical adventure.
It's sad because you want these two to be perfect for each other but they're in a way headed towards doom.
In 500 Days of Summer, Summer Finn sees her own incompatibility in her relationship in the Graduate characters. She can't express this, but the movie does it perfectly for her. She also realizes that her b/f doesn't understand the ending like she does, which makes her feel like her relationship isn't compatible like she'd hoped.