Fellatio appeals to my desire to please another --- to want to do good things for them (silly, naive... but hey, it makes me happy).
Also, I just happen to like it physically.
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen