KIRK: How we deal with death is at least as important as how we deal with life.
It's important to note that Kirk is really just mouthing platitudes here. He's going "by the book" to give the party line to Saavik. He doesn't actually believe this at all, and you can kind of sense it in the flippant way he lectures Saavik about it. Don't worry, Saavik. Our boy will learn.
McCOY: Jim, I'm your doctor and I'm your friend. Get back your command. Get it back before you turn into part of this collection. Before you really do grow old.
With his usual tact and delicacy, McCoy spells out what this trip means to Kirk: a chance to escape not a literal death, but a spiritual one. Kirk needs to be on a starship to feel alive, and if he denies that, he may as well just start fitting himself for the coffin right now.
KHAN: What do you think? They've killed twenty of my people, including my beloved wife.
From symbolic death to a literal one, courtesy of those darn space eels that will apparently eat your brain completely after turning you into Khan's loyal zombots. Proof that the villain of the piece is playing for keeps.
McCOY: But, dear Lord, do you think we're intelligent enough to... Suppose, what if this thing were used where life already exists?
SPOCK: It would destroy such life in favor of its new matrix.
The Genesis Device is interesting from a symbolic standpoint because it doesn't just kill everything on the planet. It does so while creating new life, suggesting that you can't have one without the other. That's heavy, man.
McCOY: No! You'll flood the whole compartment!
KIRK: He'll die!
SCOTT: Sir! He's dead already.
McCOY: It's too late, Jim.
This is the first time in his life Kirk has to confront the inevitability of death. Nothing he does—nothing—can stop Spock from dying, and the lesson isn't going down smooth.
SPOCK: I never took the Kobayshi Maru test until now. What do you think of my solution?
Unlike Kirk, Spock is much more accepting of his own mortality. In fact, he sees it as something very noble, something that serves the ends of benevolent logic, which he has dedicated his entire life to.
KIRK: We are assembled here today to pay final respects to our honored dead. And yet it should be noted that in the midst of our sorrow, this death takes place in the shadow of new life, the sunrise of a new world, a world that our beloved comrade gave his life to protect and nourish. He did not feel that sacrifice a vain or empty one... and we will not debate his profound wisdom at these proceedings.
This is where it really hits home—Spock's gone and he ain't coming back—but it also emphasizes that mortality really isn't the end. Spock's death brings new life. And the great circle continues, Simba.
DAVID: Lieutenant Saavik was right. You never have faced death.
KIRK: No, not like this. I haven't faced death. I've cheated death. I've tricked my way out of death... and patted myself on the back for my ingenuity. I know nothing.
A real moment of self-realization from Kirk here: the wisdom that Spock always knew but he denied, now right in his face and refusing to let go. It's a tough pill to swallow, but you can't be a hero if you don't grow or change.
KIRK: Captain's log, Stardate 8141.6. Starship Enterprise departing for Ceti Alpha Five to pick up the crew of the U.S.S. Reliant. All is well. And yet I can't help wondering about the friend I leave behind. "There are always possibilities," Spock said. And if Genesis is indeed life from death, I must return to this place again.
And with that earlier realization comes understanding and peace. Kirk can accept mortality and the loss of his friend, then move forward with a newfound appreciation of the galaxy he's saved time and time again.
McCOY: He's really not dead. As long as we remember him.
And that's kind of the point, isn't it Bones? What you do in life will be remembered by those who care about you, which is the best way to obtain immortality.