'Rough Night in Hump Junction' - Season 5, Episode 15
Alan feels Charlie is unable to control his womanizing ways.
Air Date: April 20th 2008.
Alan: Berta, have you noticed anything different about Charlie lately?
Berta: What do you mean?
Alan: Well, he's always been promiscuous, but I'm starting to think I should hide my bowling ball.
Charlie: I learned why I screw around so much.
Charlie: Yup... I think I've spent my whole life trying to fill the empty space in myself by, you know...
Alan: ...filling the empty space in others?