The Comic Book Store Regeneration - S8-E15
Raj: Wait, hang on. If you're really Nathan Fillion, what's the line from Firefly about your bonnet?
Customer: I swear by my pretty floral bonnet, I will end you.
Leonard: That's it. That's the line.
Raj: Although, I knew the line, doesn't make me Nathan Fillion.
Customer: Do you want the picture or not?
Raj: I want a picture with Nathan Fillion.
Customer: How about a picture with a guy who looks like Nathan Fillion, but a little more annoyed than Nathan Fillion usually is?
Leonard: What do you think?
Raj: Eh, it's good enough for Facebook.
The Comic Book Store Regeneration - S8-E15
Stuart: I can't believe she's gone. That woman took me in. If it wasn't for her, I would have been homeless.
Amy: One of us would have taken you in.
Stuart: Yeah, I don't recall any offers. But you know what, I'm glad it worked out the way it did, because I got to know this wonderful person.
Answer: Because Raj's selective mutism seems to be caused by women that Raj sees (on some sort of conscious or subconscious level) as a potential sexual partner. An attribute that wouldn't apply to either his sister or mother.