Although the story of (500) Days of Summer is told by a narrator in third person, we only get an inside look at what life is like for Tom. The movie revolves around Summer herself, but we, as viewers, are on our own in figuring out who she really is as a person.
At the beginning of the film, she seems like a quiet yet eccentric girl. Tom can tell she has a wall built around herself when she confesses she doesn’t believe in love. “It’s fantasy,” she says with a smile as they sit in a karaoke bar for an office party. She mysteriously leads Tom on as the story progresses, surprising him with her outspokenness at times. We see Summer’s attitude shift suddenly to unimpressed, distant, and slightly cold once she realizes he has gotten to know her. She starts to feel trapped, and before he knows it, she’s gone.
Why does Summer lead Tom on, though? If we look at it from her perspective, she just wants to be happy. She grew up in a broken home and is used to feeling nothing. “Since the disintegration of her parents' marriage, she'd only loved two things. The first was her long, dark hair. The second was how easily she could cut it off and not feel a thing." She’s constantly worried about getting hurt or hurting others, so she makes sure to tell Tom very directly that she’s only looking for a friendship. Obviously, they are more than just friends, but she is stressed about not only commitment, but the thought of getting herself stuck in an unhappy situation.
After we think Summer is long gone and Tom is finally becoming himself again, she surprises him at the park with longer hair, a more mature wardrobe, and most importantly, a wedding ring. Within 500 days, she goes from being totally against the ideals of fate to convincing Tom he was right. She says that if it weren’t for destiny, she could have chosen another place to eat or gotten to the restaurant ten minutes later, and in turn would have never met her husband. “I just kept thinking,” she says. “Tom was right... I did. It just wasn’t me that you were right about.”
Even through all of the heartbreak and time Tom spends worrying about Summer, he is able to pick himself back up. Summer, simply put, is not a mistake, but one of his life lessons.