In a relationship I masterbate occasionally, and I do it for none of the reasons you listed. Quite often I simply can't come before my partner has had enough, and we continue on to hand jobs and whatnot. I'm not at all pleased with this - I'd like to come in tandem, as it were, but I'm always taking too long. (Which can get REALLY sore after a couple of hours).

I also find it incredibly erotic to masterbate with my partner with the room very dimly lit. Something about watching her moan and girate really turns me on.

-Dan