He: I'm Nike, and you're McDonalds.
She: Why?
He: 'Cause I'm doing it, and you're loving it. :)
Nike and McDonald's Comparison
Explanation
Explain Bear
Alright, listen up, genius. This joke is playing on the slogans of two famous brands. Nike's slogan is "Just Do It," and McDonald's slogan is "I'm Lovin' It." See, it's funny because he's implying that he's "doing it" with her, and she's "loving it" like a McDonald's burger. And I know you love burgers because you are fat. Get it now, buttercup?