Take out a sheet of paper (or word processing document), take 20 minutes, and write down absolutely everything you can think of that you have any interest in doing. It doesn’t matter how big or small. Play basketball, run a marathon, climb a mountain, fly a kite, build model airplanes — absolutely anything that seems even mildly interesting to you. If you get to the end of the 20 minutes and you’ve only written a few things, then take another 20 minutes and get more creative. Expand it from activities you think may be interesting and write down activities you don’t think you’d hate.
One big problem of people who can’t find anything that they’re passionate about is that they’re overly-negative and inhibited. So they think, “Oh, model airplanes? That’s stupid.” Really? Have you done it? Why is it stupid? Why isn’t it awesome? If you can’t answer these questions without making a harsh judgment about random people, then you need to shut the fuck up and go buy a model airplane.
[Related question: why are some people overly negative and inhibited? Were they born like that? I would suggest not, and investigating how people get like that and what conditions encourage or perpetuate it would go a long way to helping individuals find what they love.]