How do I get myself sad like this?

I don't understand. Nothing is particularly wrong in my life right now. I mean at this moment. So why do I feel sad? I'm not depressed about life, I just feel like I'm missing something. & I think that missing something is someone. Or at least that's what I've convinced myself. Because I see all these other people with amazing relationships who feel "complete." And I know how it feels to like someone. I want that again.