It is our mortality that makes us appreciate being alive and when put with the belief that there is no afterlife that appreciation increases.
It is living a fulfilling and stimulating life that makes us appreciate life, death puts an arbitrary limitation on how long that can last.
That might make you further appreciate the life you live, but it is the transcience of that ability that gives it any meaning.
If it is only that which makes us appreciate life why do poor people who have no prospects of living a stimulating life cower from someone pointing a gun at them? To me it is the fragility of life that makes you appreciate that you're alive at all.
"Arbitary"??
Now, why would I, at the age of 30 want to willfully end that experience? There are things I want to do, things I want to see, people I want to help who might be struggling with this "life". In other words, there are things I want to experience.
My point is that you might feel the same way when you are 90 and on your deathbed, especially if we ignore any age related deterioration of your capacity to enjoy life. (since "getting a new body" is a common idea about the afterlife)
I'm quite confident that you won't think that the impeding end to you and your experiences will be a good thing because it made your life better.
Of course you might, but there is a big difference between dying at 30 and dying at 90. I'm at a loss as to why we ignore any age related deterioration. It is this deterioration that is part of the universe we live in. Entropy always marches on. If, for some reason, I decide to have a deathbed conversion of sorts and wish there was an afterlife, I'll be disgusted with myself. It would run counter to pretty much everything I believe and practice (my experiences) now. It would make my life, this life, utterly, utterly pointless. It would be wishful thinking and grasping at pseudoscientific bull that I despise. But it wouldn't change the fact that my life is about to end. What I believe in the last 5 minutes of my life can't alter my life in any way.
Honestly, I'm willing to be convinced that there is an afterlife if the evidence is provided but I do not actually want it to be true. I don't want there to be a God or an afterlife. I don't want the universe to work like that. If it is I'll accept it, but it will completely alter the way everybody thinks about life for the very reason that we/the universe/whatever will have removed the concept of death.
And, "enjoy reruns of Sex and the City for an eternity". Man, you might possibly have just made Hell a real place for me. 
That was my intention. 
My point was that arguments about boredom in a very long or eternal life assumes that you will stay the same.
It is like saying that a drawback of heaven will be that we will get so many children that we soon won't be able to remember even a fraction of them. This is a poor argument because heaven doesn't necessitate the existence of sexual reproduction,a lust for sex or a desire to have children.
One could make the argument that a continuation of your life necessitates that these aspects of your personality must stay the same, but I don't see how these kinds of chances would put an end to you as a person more than behavior modifying drugs does.
In an eternal afterlife it wouldn't matter if you stayed the same or not. Eternity is forever. Everything you do you would do again and again and again and again.......
You could do anything you could possibly think of and a whole lot more you possibly couldn't and you still wouldn't be anywhere near the end of it if you did it over and over and over again. In eternity the beginning is as close to the end as the middle is. There is no frame of reference.
A "very very long" time wouldn't even be noticed by someone/thing experiencing eternity btw.
Sure it would.
10 minutes of euphoric bliss is 10 minutes of euphoric bliss.
Three days of extreme torture will be about as unbearable no matter what fraction you expect that these three days will be of your remaining time alive.
Experiencing the moment will always exist and how enjoyable this experience is will always be a primary force in driving our current mood. Reasoning about the past and the future is a sort of metacognition that matter, but it is more of an added layer than the foundation for our experiences and the resulting mood.
I'm not saying you wouldn't be aware of it while you are doing it, just that in the overarching narrative of eternity it wouldn't even be a blip on the landscape. You could be tortured for a billion years and to you, an eternal being, would forget that it happened at all.
We really don't have any notion whatsoever as to how we'd deal with eternity, but I'm pretty sure that the reasoning and emotions we think with in a
finite time frame would be next to useless.
“The world is so exquisite with so much love and moral depth, that there is no reason to deceive ourselves with pretty stories for which there's little good evidence. Far better it seems to me, in our vulnerability, is to look death in the eye and to be grateful every day for the brief but magnificent opportunity that life provides.”
― Carl Sagan
To me it doesn't seem as if he is saying that "looking death in the eye" is what makes him see so much love in the world or appreciate the opportunities that life has.
I read it more as "the fact that this awesome life will end still leave us with some awesome life".
I don't know where you're pulling that last bit from. I read it as that it is because it is fleeting that it is precious.
OK then try this:
“You have to give up! you have to give up!
You have to realize that someday you will die,
Until you know that, you are useless!”
