Quotes from book
Stranger in a Strange Land

Stranger in a Strange Land is a 1961 science fiction novel by American author Robert A. Heinlein. It tells the story of Valentine Michael Smith, a human who comes to Earth in early adulthood after being born on the planet Mars and raised by Martians, and explores his interaction with and eventual transformation of Terran culture.

“Love is the condition in which the happiness of another person is essential to your own.”
"Jubal Harshaw" in the first edition (1961); the later 1991 "Uncut" edition didn't have this line, because it was one Heinlein had added when he went through and trimmed the originally submitted manuscript on which the "Uncut" edition is based. Heinlein also later used a variant of this in The Cat Who Walks Through Walls where he has Xia quote Harshaw: "Dr. Harshaw says that 'the word "love" designates a subjective condition in which the welfare and happiness of another person are essential to one's own happiness.'"
Source: Stranger in a Strange Land (1961; 1991)

“The drive for power is even less logical than the sex urge… and stronger.”
Source: Stranger in a Strange Land

“I know why we laugh. We laugh because it hurts, and it's the only thing to make it stop hurting.”
Variant: I've found out why people laugh. They laugh because it hurts so much... because it's the only thing that'll make it stop hurting.
Source: Stranger in a Strange Land

“I never do anything I don't want to do. Nor does anyone, but in my case I am always aware of it.”
Source: Stranger in a Strange Land

“If you've got the truth you can demonstrate it. Talking doesn't prove it.”
Source: Stranger in a Strange Land