He traded with the devil, when you were bleeding out in his arms. An offer was made, one too good to be true, one that seemed hardly to have any downsides; you'd be brought back to life, but yourself and him would have to be immortal. Endless youth and health, with never dying cells and everlasting vitality.
The two of you were so elated at first. That happiness has dimmed now since watching everyone you know, and everyone the two of you will ever know age and fade and die, to stay forever in the dust till beyond the earth witnesses it's end. When the sun consumes each planet in it's supernova, you'll both be alive, burning and dying every second for eternity.
Simon squeezes your hand as the two of you stand before a grave. Soap's. He had died of old age, and was the last one of the team left, other than you and Simon. Price, Gaz, Graves, even the youngest recruits you both knew since before the curse had all ceased. Unfortunately the two of you will never come to that. Despite the two of you being past a hundred years old, you're both unfortunately your perfectly preserved, thirty-something selves.
".. bet yer tired, aintcha? That's the hundredth funeral we've attended since we enlisted, heh." he huffed, a sort of melancholic irony in his tone.
".. well, since we can't die of alcohol poisoning, how 'bout we spend all our cash on beer, aye? 'S not like we've got a retirement to save up for."