People in Your Life

I was thinking, and I noticed all the people that go in and out of your life. Like if you have a teacher for a year and then they move, you never see that person again. How depressing is that? Do you ever think of those people? Do you remember the times that you spent with them? Do those people have an impact to your life for the short time that they are apart of it? Was it by choice that those people are no longer in you life, or did you have absolutely no say at all? How do you deal with the so called "lose" of that person?