So you’re telling me it’s NOT because they like to crawl into your ears while you’re asleep?
If you want to lose the solace that fact has provided you, here’s another possibly also false but no less comforting bit of trivia:
spoiler
Leeches after feeding would love nothing more than a dark, somewhat moist, somewhat warm, somewhat tight environment. Which a human has and is probably something you don’t want a leech to get anywhere near, be glad there’s no such thing as a Analwig. Oh and land leeches exist.