I'm not sure that this statement is necessarily true for adults, but it might appear to be true for teenagers. Your family is your family by decree of birth. Your friends choose to be your friends, but your family doesn't choose you. By this same theory, teens might believe that family have to love and support them, but their friends choose to be their despite the flaws.
Friends can end up being more important than family members, especially as a person moves into their teen years and beyond. This is because friends are from a person's peer group and therefore their opinions tend to be more relevant to a person during those years.
When a person is young, they get their main support from their parents and siblings. They rely on those people for emotional support and for socialization. But as they get older, they become more interested in what their peers think of them.
There are at least two reasons for this. First, they will need to form lasting relationships (including, presumably, romantic relationships) with people of their own age. For this reason, the opinions of people their own age matter a great deal. Secondly, family is a constant thing. Your family has to love you no matter what. People in your peer group are not like that. You have to worry about how they will perceive you and you get more (at times) pleasure from their acceptance because it is not automatic.
There are other reasons for friends to take on primary importance in a person's life, and they have to do with the nature of friendships and the nature of family, that is, what defines these relationships and how they form.
We are born into family, of course, and we do not get to choose those we are related to. Often times a person will have little in common with their parents or siblings (even at times, I have found, in the case of twins), but are nevertheless bound to them by blood relation, economics and geographic location, not to mention by law.
As we begin to mature socially, we are able to choose those we associate with, and human nature leads us to choose those who we share activities, experiences and opinions with. We choose those who reinforce our world view and reassure us that we are normal and accepted, whether we realize we are doing this or not.
For this reason, we tend to have more in common, are closer in age to, and can relate more to those we call friends than those we call family, and we begin to spend more and more time with friends than family as a person enters their teen years, and even more so after we move out of the house or go off to college. Then our primary group of friends replaces our primary group of family in terms of everyday socialization.