Boy meets girl, boy loses girl. It’s been done to emo death. That’s why the sublimely smart-sexy-joyful-sad (500) Days of Summer hits you like a blast of pure romantic oxygen. It turns the genre on its empty head and sees relationships for what they are — a bruising business. Someone’s heart always gets ripped out. Meet Tom (Joseph Gordon-Levitt, letting his charm flag fly), a would-be architect idling in L.A. as a writer of greeting cards. In strolls his boss’s new assistant, Summer (the glorious Zooey Deschanel), and Tom’s a goner. For her, prizing independence, their affair is a lark. For him, raised on sappy Brit pop and a total misreading of The Graduate (he thinks the ending is happy), it’s love. So when she walks away, firmly but with kindness, you feel the pain, the kind that evaporates quickly only on sitcoms.