Be kind because you want to, and if you don’t want to, be kind out of spite that you were born without the desire to want to. The latter is for you rebellious people out there. Nothing is more rebellious than the rebellion waged against your own predisposition.
You’ve evolved to a state of social dependency for a reason. Yes, this may sound a little cold and calculative, but so is the instinct to eat and you still do that right? So be kind out of self-preservation at the least. We often need people in our lives to help us out.