The main difference here is the choice. We choose friends based on common interests and similar views. Usually, you do something together, for example, keep to a healthy lifestyle. These are people who we are comfortable and easy to be together with. If you loose interest in somebody of them or your interests diverged, you can just go your separate ways peacefully.
Relatives is a different story! Like it or not - they are with you forever! You can't just get rid of them, no matter how annoying they are. This is the challenge for all your life . Yes, don't be surprised, this is the point of your growth. That, what doesn't kill us - makes us stronger.
That’s why movies about big families are so funny . You see yourself in these films, and different psychotypes of people from your environment. And you smile observing them, because you know how it’s difficult to communicate, when all just you want is to run from this place. And you are sure that next time you will have to kick off all over again. :)