That´s kind of a scary question because it makes me feel you are picturing yourself as a sociopath. 
 
Anyway:
Because our wellbeings are interdependent, to boot - so we 
always benefit indirectly.
Secondly, because we just do.
On another note, I am assuming that finding a good way to coexist has always been the 
motivation for moral/ethical considerations (and appeals to existing or non-existing alleged authorities have always been 
means to that purpose, especially effective in times of authoritative thinking). It´s not like our lives are to serve morals, it´s more like morals were and are invented to serve us and our wellbeing.
This meta-moral question always strikes me as similar to "Why 
should we eat?".
But if that which is blatantly obvious to me (living in a world that is enjoyable is a value, and contributing to creating such a world is therefore in my own best interest) doesn ´t resonate with you - maybe 
you want to share 
your own answer to the question you have raised?
On a final note, your question can be (accordingly) asked ad infinitum, no matter what the answer is.
E.g. if the answer is "Because God says so.", the next question would be "Why should we do what God says?" (and so forth).