Have you ever made a sandcastle near the ocean shore at low tide, knowing that it would soon be erased by the waves? It probably happens thousands of times every day. Kids (and kids at heart) carefully craft the moat and courtyard, drizzle a spire of wet sand on each lofty tower, stand back to admire their work. All while keeping in the back of their mind a solemn understanding that all good things must come to an end.
Of course, it may not even occur to the youngest beach builders that their efforts will be washed away. They work with such determination that the incoming surf takes them by surprise. When the water finally crashes through their frantic attempts at a defensive wall, they can only watch and mourn the ruins of their once-mighty fortress. But even for these naïve castle-makers, their sorrow at the destruction of their castle does not outweigh the satisfaction they got from creating it. If you asked them, most would say it was all worthwhile.
Those who believe in eternal life sometimes wonder how the rest of us can live with the fact that it's all going to end someday. Even if we could use technology to achieve biological immortality, we would still ultimately be limited by the heat death of the universe in roughly 10100 years. So, they ask, what's the point of trudging along each day if it's all futile and meaningless in the grand scheme of things?
To which I answer: Why build that sandcastle?
Because you enjoy it while it lasts. Because you treasure the memory as long as you can. Because its very impermanence is what makes it so special.
Further, I put it to them: What is it about the prospect of eternity that imbues our existence with meaning? I don't see how the mere existence of an endpoint in any way negates our current actions, or how the lack of one is needed to validate them. In fact, the more you start really thinking about what really eternity means, the harder it is to imagine it as anything other than a fate worse than death. If you lived for another 101,000 years, you'd probably be too busy going mad with boredom to think back on how significant your life was 101,000 years ago.
Savor your life in the here and now—everything, from your fast food burger to your wedding day. If not for you, then out of respect for all those who will never get to. Because out of the countless quadrillions of people that could have been born to live a short life on this little blue planet, you are here.
You are here to gaze up at the stars and ponder your kinship with the universe. Be glad that you can reflect on the past, relish the present and make your mark on the future. And even though that mark will eventually be washed away in the waves of time, be grateful.
Be grateful, because you didn't have to be here—but you are.