"Blood is Thicker Than Water"?

I've heard it said often that you should value your family higher than you value your friends. The reasoning for this, or so it seems to me, is that you can pick who your friends are, but not who your family are. Under scrutiny however, this seems to be a case against that conclusion, rather than supporting it. I mean, if the family that I am born into is nothing more than chance (actually if you want to get technical about it then it isn't really chance, but lets not get into that) whereas who you are friends with is something you and another person consent to, verbally or otherwise, then it seems logical, to me at least, that that is the relationship that I should value more highly and treat with greater care. Don't get me wrong for a moment, I love my family. Of course there are some branches of my family tree for which I couldn't claim to be unhappy should they be, how should I put it, pruned, but for the most part I am happy with the family that I was born into. I just feel that if I want to be friends with someone, and that someone wants to be friends with me, then that is someone who I feel should come a little higher on my priority list than someone for whom a relationship with was something that I had no control over.

Lets take the example of marriage. Marriage is like friendship in a lot of ways, except it is essentially pronouncing your friendship on a public stage. Now if you are married to someone (let's not get into family inbreeding) then most people would think it is perfectly acceptable to treat that person with more devotion and care than anyone else. I can't see why it would be incorrect for me to apply the same logic for everyone that I have chosen to have a relationship with, and they with me.