I don't think I've fallen in love since elementary school, though I've had a few crushes and infatuations since then. In elementary school they were both people in my classes, but I think it's possible to fall in love with a stranger. The essence of a person is simple--you don't fall in love with a collection of psychological facts about the person--and if you're perceptive enough and/or they're effective at expressing their true selves enough you can fall in love with someone immediately. Also, celebrities entail one-sided relationships. To them you're a stranger, but to you they're very familiar, so in that respect you can fall in love with a stranger.