Friendship has to be earned and you have to work at it!
You don't charge or expect money for favors from your friends, but your friends aren't simply there to listen to you ranting on for hours on end, to run to the crèche and pick up your children when you can't make it or drop everything to help you cope with whatever disaster has befallen you.
So we have to think about thanking them and making sure we're there for them if they need us. Friendship isn't a given and if you don't maintain it you can lose it. It's very important and very much appreciated when you tell them how much they mean to you.