Strange that we promise to be someone’s best friend in exchange for their doing us a favor. Being someone’s best friend is easy. It requires only that that other person do us a favor and then place us above all their other friends. It’s not even exclusive. You could go to 10 people and promise to be their best friend if they do something for you, and there’s no contradiction because you could be their best friend, so long as they’re not all your best friend.

“You’ll be my best friend if you…” is a much more balance deal, since it’s an actual give and take. Not that inadvertent semantic reversals ever got in the way of language evolution. I’m sure a lot of people (myself included) could care less about it. Could you?