Some people come in, stay for a while, and then leave once you've learnt the thing/things you need them for.
Some people come in, stay for a longer time, because they have more to teach you or because their thing takes longer to engulf you.
Some people come in to your life, teach you something, leave, then come back.
Some people come in and stay forever because they are forever growing and forever changing along with you. They stay because they want to, not because they are forced to.
I am lucky enough to have some people in my life who are from all of these categories. Everyone is lucky to have people around them; be it your family, your friends, your colleagues, people you meet one day and then never see again.
We all need people. As much as I may say that I am fine being by myself a lot of the time, I do need people. I need human interaction and I think it's important that everyone remembers that. It's what keeps us human, it's what keeps us ticking. Humans are sociable people. Even if you socialise for 5 minutes in a day, that's what really keeps you ticking.
So next time you're out and about, or next time you're lying in bed thinking... think of the people who come and go, think of the people who stayed for a while, think of the people who came back and think of the people who will never leave. Be thankful for them. Without them you would never be the person you are today. You'd never know the things you know now. Humans are forever growing, learning, changing and adapting. Through pain, sorrow, loss, happiness, excitement, nervousness. Emotion is how we grow, learn and adapt.
I believe everyone comes in to your life to teach you something, or to help you become the person you are meant to be.