Who said "I don't understand why everyone's so serious all the time."?

This quote is attributed to Billy.

I don't understand why everyone's so serious all the time. - Billy

"I don't understand why everyone's so serious all the time."

Billy Humor

"I'm a big fan of science!"

"Mandy, wouldn't it be great if we could stay young forever?"

"I love the smell of destruction in the morning!"

"Why does everything have to be so complicated?"

"Never underestimate the power of a good laugh."

"Laughter is the best medicine."

"A good laugh is the best medicine."

"Laughter is contagious and wonderful."