Everything has come into being for a purpose - a horse, say, a vine. Does this surprise you? Even the sun will say, 'I came into being for a purpose': likewise the other gods. For what purpose, then, were you created? For your pleasure? Just see whether this idea can be entertained.
Nature's aim for everything includes its cessation just as much as its beginning and its duration - like someone throwing up a ball. How can it be good for the ball on the way up and bad on the way down, or even when it hits the ground? How can it be good for a bubble when it forms, and bad when it bursts? A candle is a similar example.
Without commenting on whether the soul is eternal or not, one has to agree with Marcus with regard to physical things. All things die and change states. God says so much in the Bible, "you are dust, and to dust you shall return" (Genesis 3:19). You can do nothing but accept this fate. You have absolutely no control over this.
You have to ask yourself what your purpose is. It takes a lifetime for some to find out. For others, they know what they are supposed to do. But philosophically, you must ask yourself if you exist to only eat, sleep, pee and poop or if there are more interesting things you were meant for. What makes us humans unique is our brain, our capacity to act and be creative. I think somewhere in that space you and I can find our purpose. If nothing else, find a person or a few people who you love and make their life better for having known you.
Marcus, reminds himself, again, that all things die and that there is no need to be upset by these things. A ball goes up, it comes down. A bubble forms, it pops. A human is born, he dies. I too will repeat myself ... You can do nothing but accept this fate. You absolutely have no control over this.
(see also Citadel p. 43, 270)