"Family love" is taught and ingrained into us from a very early age. Witness a 2 year old child on the phone with his/her grandmother, more often than not the call will end with the parent of that child coaxing them to say, "I love you grandma".
Clearly there is NO WAY the child knows what those words mean nor does he have that "feeling". As the years roll by we "learn" family is to be loved, "blood is thicker than water" "appreciate and respect" your parents for "bringing you into the world"...etc
As Jarn stated in his answer we love family out of "obligation". It has also been said, "Friends are the family WE choose"