On the most basic level, humans are biologically selfish. Physically our body builds immunities. Through natural selection we have a temple that is designed to protect us from illness. What about mentally or emotionally? We are biologically selfish because our brain doesn’t just try to protect from physical harm, but also emotional harm.
For centuries humans have found emotional solace in company. Our sexual partners, our family, and our best friends for example. When a mutual connection is made, it’s not selfish, just two humans easing each others emotional needs.