Amy: Can you please just tell me why?
Sheldon: Fine. As you know, I have a twin sister with whom I obviously share a birthday. Every year we'd have a party. No one I invited would ever come, because they didn't like me.
Amy: I'm sorry.
Sheldon: Oh, that part wasn't so bad. I didn't like them, either. But then I'd inevitably spend the whole day being tortured by my sister's friends.
Penny: Oh, you poor thing.
Sheldon: When I was six, they told me Batman was coming to my party. I waited by the door for hours. Closest thing to Batman I saw was when a robin flew into the window.