Why did Michel Gondry, the director of Eternal Sunshine of the Spotless Mind, make this wee-size, super-nice documentary about his Aunt Suzette, who was once a devoted grade-school teacher? You can see that he meant it to be a toasty slice of nostalgia — and for Gondry, it surely is. For us, The Thorn in the Heart is more like a home movie of a family reunion we weren’t invited to. Suzette Gondry, all white hair and laughing memories, seems like a lovely person, and there are a few moments of tension concerning Gondry’s older gay cousin. But overall, this lazy, sweet trifle seems to express the banality of well-being. C?