His eyes.

He commanded me one day to look at Him, and He gave me a Look which immediately flipped a switch in my brain. We have had a BDSM relationship ever since, and the Look still is an instant subbie switch which makes me completely useless for any function that doesn't involve His manhandling me. Actually, much like I "knew" that my husband was The One, I knew that my Master was The One. I don't mean to be romantic or unhelpful, but I have always felt an immediate and complete Trust of Him, that I do not feel with anyone else. I know that when we are together, we are so much more than when we are apart, and we are loving life right now :-)