Because you have to live with yourself in the end. If it makes you feel like a good person to be kind, then if only for that one reason, it is the right thing to do. Even if most all people in the world become selfish. and totally unkind, if there are a few remaining that are genuinely kind, have empathy for others and are against harming others, there is still hope for mankind. If there were to be absolutely no more kind people in the world ,there would be no more hope for humanity. So, as I tell my teens, be kind, even when no one else is. Pick up litter someone else threw out( for the same reason. If NO ONE picks up the litter others throw out we will all be living in a landfill soon). Be the better person, because humanity needs the people that rise above what all the other people are doing. So be kind for the sake of humanity as a whole, not for the one on one interaction with other humans, because those interaction may be less than kind. Here is one last thought. If you be kind to someone that wasn't kind to you, if it touched their hearts, they will perhaps be kind to the next person they meet and you will have made a difference.Besides what society deems rational,has not had a good history of being what was right.