The Big Bang Theory, Season 7

Professor Proton:
Can I ask you a question?

Leonard:
Yeah, sure.

Professor Proton:
Why do you put up with Sheldon?

Leonard:
Oh, um, you know. Because we're friends.

Professor Proton:
Why?

Leonard:
Wow. You ask really hard questions. Look, I know he can be aggravating, but you have to remember he's not doing it on purpose. It's just how he is. Oh, but he's also loyal, and trustworthy and we have fun together.

Professor Proton:
You know, you're describing a dog.

Leonard:
He did bite me once. But in his defense I came up behind him while he was eating.

Professor Proton:
They hate that.

Leonard:
You, know what. Sheldon is the smartest person I've ever met. He's a little broke and he needs me. I guess I need him too.

Professor Proton:
Why's that?

Leonard:
You will not let this go.