Why some people can't just stay in our lifes

So. I recently (and many many times before that) thought about some people that came in my life that I wish I could keep longer but I can't. They come with a purpose, like I want to have my driver's license now and I wish I could keep my teacher in my life but I can't. He comes only to teach me and then disappear like other people did. But we do click. I'd like to have him in my family as an uncle or something. Or a woman I met somewhere I was working. I felt so close to her. Barely had thoughts like.. what if she was a sister to me. Or cousin.. we shared similar thoughts one two times. And then we stopped talking. I mean she has a family she's 10-15 years older than me. But why we can't hold some people with us.. why we have stupid people surround us and some people we really really like (in a human way) we have to let go sooner or later. Just to make it clear I'm not talking for friendship. Just relationships with people you feel comfortable like family (real family) but their not so you have to let them go again