Killing it...

Toothache!

Well I can only speak for masturbation but a few things do it in for me.
Our dog deciding to jump up on my chest is instant murder for the 'mood' no matter how good a toy I may be using.
But I can't lock him out of the bedroom because he starts howling. =_=;

This.

Ugh, when he is constantly wanting to change positions or tries to throw me into a position that he knows I hate. He gets too excitable and just totally forgets what works and what doesn't.

Then again, our 7 month old crying in the next room does suck the fun out of it pretty quick too lol