Because they can. It's part of their biology.

Are you making fun of me?

Well sure Vietnam was fun, but not like going to the circus fun, or fly fishin' in Montana fun.

You have fun.

You're fun. A toast to fun.

Welcome to the age of "un-innocence."

and then sittin' in a tub with tobasco sauce fun.

...Ieft Charlotte exhilarated and happy as she'd been in ages.

Girls, isn't this fun?

...there's a very powerful part of me that connects to the female spirit.

Cartman! What kind of stupid ass question is that. Of course it was fun!

the real fun begins.

This was fun.

It was fun.

You're part of a dialogue.

That wasn't part of the plan.

- Let's hunt. - We'll start with a 2-shot of you and Ned getting your equipment together and..

Have a fun day.

They were fun, smart...

Relax! It'll be fun!