I have seen this attributed to CS Lewis. I seem to recall reading the source of this quote, but I have forgotten which book or essay it was.
'The rule for all of us is perfectly simple. Do not waste time bothering whether you “love” your neighbor; act as if you did. As soon as we do this we find one of the great secrets. When you are behaving as if you loved someone, you will presently come to love him. If you injure someone you dislike, you will find yourself disliking him more. If you do him a good turn, you will find yourself disliking him less.'