his may sound trite, but the answer is loving them:

My first partner, whom I could not marry at the time, told me that our relationship was the best thing that ever happened to him, and expressed gratitude for my being there with him during his final illness.

My current husband tells me that had he not married me, he’d most likely be sitting at home alone, rather than singing in a chorus, and having a much more active social life with our friends.

So, at the risk of sounding immodest, the best favor I’ve done for someone was committing to sharing a life with them.