I agree, if you "like" your opponent you will find a way to lose.
Fortunately for me, my pool sparring partner (who is also a good friend) get's upset with me if I don't always play my hardest against him whenever we play. Makes for good practice sessions.
As for competition. If someone shakes hands with me just before a tournament or league game and says something like, "Please, take it easy on me and don't beat me too bad." I smile and think to myself, "Red alert. This person will stick a fork in you the first chance they get. No mercy. Be nice AFTER the game is over."