Among evolutionary psychologists there is this view, which I share, that empathy (not sexual passion), is tied to the amount of DNA in common. The more your genetic structure is the same, the more you feel empathy for the other. That does not mean that you necessariy love the other, but having DNA in common is an element that weighs in the relationship. So:
Mother – her child
Twins
Brothers
Blood relatives
Humans
Mammals (particularly chimps, apes)
Other living critters along a ladder
There is this expression often used: “blood of my blood”, “flesh of my flesh”.
Children who have never known their fathers (it happens more rarely that they have never known their mothers) that start searching for their parent at some point in their life, and think that it is important, also if they are normally disillusioned.