The Communication Deterioration - S8-E21
Penny: You are a wise man.
Sheldon: Well, Penny, who's smarter, the wise man or the person who comes to him for advice?
Penny: Oh, I guess you're right. Maybe it is the person who asks.
Sheldon: No. It's the wise man. That's why he's called the wise man. You know how I know that? I'm the wise man.
The Colonization Application - S8-E17
Amy: We could also be the first people to procreate on Mars.
Sheldon: You just can't keep it in your space pants, can you?
The Intimacy Acceleration - S8-E16
Penny: If you were to die this evening with no opportunity to communicate with anyone, what would you most regret not having told someone?
Sheldon: So it would be today? Huh... Well, I suppose there's something satisfying about dying on my birthday.
Penny: Today's your birthday?
Sheldon: Yes.
Penny: Well that's always been a secret. Not even Amy knows.
Sheldon: Well, I don't enjoy presents. And the thought of people jumping out and yelling surprise fills me with more dread than the words "George Lucas Director's Cut."
The Intimacy Acceleration - S8-E16
Sheldon: Just out of curiosity, Penny, if this experiment does make us fall in love, would you drive me to Lake Geneva, Wisconsin, for Gary-Con? It's the only convention celebrating the life and work of Gary Gygax, the co-creator of Dungeons and Dragons.
Penny: You know what, I can honestly say if we fall in love, not only will I drive you there, I will buy you all the dragon T-shirts you want.
Sheldon: Okay, babe, let's do this.
The Intimacy Acceleration - S8-E16
Bernadette: You better find my husband's mother, 'cause one way or another we're walking out of this airport with a dead woman.
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.