I definitely agree -- I get the most turned-on when I see that my partner is enjoying what's going on. Usually during sex that's what I'm most focused on, too. However, I find that there is definitely a power dynamic inherent in giving pleasure, most evident during such non-traditional sex acts as pegging... At least for me, it is a heady sensation to know that I am actively giving pleasure, rather than just reacting to it being given to me.