Friendships, as a rule, come and go.
Some come and go quicker than others.
Pay attention to those that stick around, though.
They might not always be the people you spend the most time with or consider yourself closest to, but occasionally you'll find that there are people who'll stick by you for years, keeping an eye out for you. Those're the people who'll have your back when you need them.
When the going gets tough, you find out who these friends really are.
But don't wait for that.
Appreciate the friendships you have. And not just the obvious ones.