I've been on both sides of this and below is just one example:
A friend wanted to go to a specific restaurant at a specific time. He invited me and another mutual friend. So it would have been the 3 of us. The mutual friend cancelled. I decided to cancel to, especially if the third guy wasn't coming. I feel a bit guilty for doing this. Is it fair to cancel because another person cancels, or is this selfish? Some reasons I would do this are because 1) I am really coming to socialize and visit with a friend 2) we were going to share the meal and it may have been too big for two people.
OTOH someone pointed out to me it's rude to only want to hangout with someone on the contingency someone else it there, because that's kind of showing they aren't really your friend.