(500) Days of Summer may not be a particularly happy love story, but it's a love story all the same.
Oh, reckless abandon,
Like no one's watching you.
-- The Temper Trap, "Sweet Disposition"
(500) Days of Summer begins by telling us what it's not. A title card asserts the characters are not based on anyone real. The narrator warns, "This is a story of boy meets girl. But you should know up front, this is not a love story." Of course, it is exactly that. It may not be a particularly happy one, but it's a love story all the same. Happily, it's a love story with a cool tone and terrific performances, as well as a perfectly matched soundtrack that is not used as a substitute for plot.
The movie opens with a break-up. The partners are Tom (Joseph Gordon-Levitt), whose firm belief in "true love" makes him seem well suited for his job writing greeting cards, and Summer (Zooey Deschanel). When she starts working in his office, every man in the room is smitten, but Tom recognizes her as "the one." We already know (from the narrator) that Summer, damaged by her parents' divorce when she was young, thinks love is a fantasy and has no interest in any relationship that is not safely casual.
The film examines their 500 days together from Tom's point of view. So, even though the narrator suggests that his romanticism is misconceived ("This belief stemmed from early exposure to sad British pop music and a total misreading of the movie The Graduate"), he remains our hero. We're invested in his project, which is to sort through his memories to find the moment when things started to go wrong.
On Day 500 of their relationship, as Summer dumps Tom over pancakes, she is astonished that he hasn't seen it coming, since they've been "fighting like Sid and Nancy" for months. When he recoils at the idea that she would see him as someone who might harm her, she corrects him, saying, "No, I'm Sid." And therein lies Tom's real problem. It's not that he is believes in love, it's that he's allowed Summer to feminize him. Likewise, though Summer's resistance to intimacy is understandable, she holds too much power in their relationship.
It's in this light that we see Tom (and Tom sees himself, remembering) as he obsesses over Summer's every word and action, inadvertently forfeiting the first move to her (and, for that matter, ever move after). When they finally hook up, he breaks into a dance through the park, to Hall and Oates' "You Make My Dreams Come True" (which everyone he meets joins in on, including a cartoon bluebird). The scene is telling. Though undeniably fun and cute, it reinforces Summer's view of love as fantasy and makes Tom look distinctly feminine, very nearly playing Amy Adams in his own version of Enchanted.
As Tom starts off feeling devoted in his relationship with Summer, he can only act out in overly emotional (that is, typically feminine) ways when they break up. When he starts breaking dishes and crying to his friends, they play typical best girlfriends' parts, assuring him that Summer doesn't deserve him. Whether together or apart, Tom is reduced to observing her, alternately adoring or loathing her hair, eyes, and laugh. This fixation, like the dance scene, works on two levels: Tom may feel like a spectator to his own life, but in his memory, Summer takes on a new role -- not the dominant partner, but the observed, powerless before his gaze. Even the film's poster enforces this objectification, with hundreds of little pictures of Summer's face or parts of her face.
Given this structure, it would have been easy for (500) Days of Summer to vilify Summer as a she-monster: thankfully, it doesn't. While Tom's perspective is certainly privileged over hers, a balance is struck when he finally gets his chance to question her on why their relationship has failed. This moment is honest and not a little crushing (and Gordon-Levitt, it must be said, plays silent heartbreak to perfection). In recognizing that his recollections are only his version of the truth, he sees as well that Summer's version, no matter how different, is no less true.