the only thing you can love is yourself, so in order to love someone else you need to make them a part of you.
Your true self is the decisions you make to overcome your nature.
So by giving, you are overcoming your nature, and building yourself.
But when the giving is brought on by the other, they become part of your self, thus you love them.
This is the same reason we love a cake we make much more then a cake we bought.