One of my favorites was Requiem for Methuselah mostly for McCoy’s speech to Spock at the end.
“MCCOY: Well, I guess that’s all. I can tell Jim later or you can. Considering his opponent’s longevity, truly an eternal triangle. You wouldn’t understand that, would you, Spock? You see, I feel sorrier for you than I do for him because you’ll never know the things that love can drive a man to. The ecstasies, the miseries, the broken rules, the desperate chances, the glorious failures, the glorious victories. All of these things you’ll never know simply because the word love isn’t written into your book. Goodnight, Spock.”
“You know why you’re not afraid to die, Spock? You’re more afraid of living. Each day you stay alive is just one more day you might slip, and let your Human half peek out.
That’s it, isn’t it? Why, you wouldn’t know what to do with a genuine, warm, decent feeling.
Spock: Really, Doctor?
Dr. McCoy: I know. I’m worried about Jim, too.
-—Bread and Circuses