Do you honestly believe everything humans do isn't done out of self interest?

EVERYTHING we do ultimately comes down to trying to spread and preserve

* our genes * our family's genes * our friends' genes

and all the other social groups humans form, in the order of priority being group size smallest to least.

Emotions are the definable channel to that "gene preserving" desire.

Everything we do in life is to achieve a certain emotional state (think about it). We don't want money. We want what it will mean to us (e.g. security, power, fun, freedom, etc.). We don't want a relationship, we want what it will mean to us (love, belonging, etc.).

Nature's calibrated human has his emotions correctly / optimally linked to spreading and preserving his and other humans' genes.