I think a huge part is the fact that evolutionary wise we needed to be part of a group in order to simply survive: being able to get food together and protect each other. I guess we forgot all that along the way.... Nowadays it helps us feel less lonely, maybe even helpful to others, and it makes us happy to feel seen by our peers. Next to that there's something undeniable about the connection we have to our parents, siblings, other family members. Even if one or both sides are "not interested", it has huge impact on who we are, who we feel we are, how we behave and see the world... maybe with hatred or sadness etc, but it absolutely does affect us is we sever that bond with our family.