I like to mix it up.
"Trust me, I'm a doctor/police officer/organ donor/alcoholic/gypsy."
"Don't worry, I've done this plenty of times before in video games."
"Because it was funny."
"I refuse to answer on the grounds that I don't like you."
"Would you believe, spontaneous temporary insanity?"
"Tucker did it."
"Because I fucking can."
"I blame Jack Thompson."
"It seemed like a good idea at the time."
"I swear to Drunk I'm not God."
"Why do you hate your country?"
"You never specifically said I couldn't."
"Because shut up."
And my personal favorite, a sure-fire answer to someone asking "What are you doing?":
"Yes."