Every child on Privet Drive, Surrey Place, England knew three essential things: the earth revolves around the sun (courtesy of Mrs. D, retired physics teacher and still avid educator), ice cream tastes best after a long afternoon of play, and everyone has a soulmate out there in the very big world. Of course, the precocious few of the lot knew a few more things (the average weather for each part of summer, as Bobby will no doubt be happy to tell you), and the less precocious but no less adorable hold the same belief. Facts for children follow little logic: the sun sets because it does, grandmum prays because she should, and the Dursley family is the most abnormal of the block, presumably because of Harry Potter.
Because they were children, they also knew (usually absorbed from the world around them, though some parents did try to explain something unexplainable) that something magical would happen on their seventh birthday. Something utterly amazing: they would receive a vision, like a video inside their heads, of best moment of their soulmate's life. They didn't spend too much time thinking about it, because really, even if soulmates were supposedly devoid of cooties, romance was still pretty gross.
One child on the block knew little of the other families' standards of normal, growing up alone and sheltered, but he did know some things the other children of privet drive knew nothing of: sometimes, if you wished hard enough, things happened. It wasn't magic, of course. Magic wasn't real, first of all, and second of all, if it were, he would be happily living anywhere other than his cupboard.
But as his seventh birthday grew near, Harry stayed up and thought about his soulmate. He wasn't quite sure what a soulmate was—his aunt and uncle were less than accepting of "unfounded voodoo," as they called it—but Harry was sure that his soulmate was going to be the one person who'd truly understand him. Unlike the Dursleys or the other children, he wouldn't believe Dudley's lies. He wondered what his soulmate's best moment would be. For some lucky people, it was the day their soulmate met them. For others, it was their soulmate's wedding, or graduation, or another happy thing.
When the clock struck twelve, Harry's body fell onto his bed while his mind dove into the future. Eventually everything stopped spinning, and Harry found himself in a bathroom, standing next to someone he guessed was his older self. He was a teenager with a lightning-shaped scar on his forehead, and Harry was confused until he saw the other person in the room. He was the older Harry's age, blonde, dressed in the same baggy black dress as the older Harry, and had a wooden stick pointed straight at him. And then he spoke a strange word, and both Harry's made the same expression of pain.
It hurt. It hurt even when the vision stopped and he came to himself. Harry huddled into himself, phantom pain still running through his body, and decided he really didn't like his soulmate. How could he, if hurting Harry was the best moment of his soulmate's life?
The only time Harry ever spoke about the blonde boy was to Hagrid, and when Hagrid replied with, "That's yer soulmate! But you shouldn't be talking about this with me, you know. You get to know the worst moment of your soulmate's life for a reason, so that when you two meet, there's no secrets."
Harry nodded, and wasn't comforted when Hagrid talked about the worst day of his own soulmate's life, which was when her dog died right in front of her. At least Hagrid's soulmate wasn't a blonde boy who'd curse him terribly one day. Even if cursing Harry was going to be the worst moment of his soulmate's life, he was still going to do it. Visions couldn't be altered.
Sometimes, Harry really would've rather been a muggle, and seen the best of his soulmate instead.