Why
Explanation
The Joke
A man lies in bed unable to sleep, praying to God: "Dear God, I can't sleep. I keep wondering... how come I'm poor, alone, weak, confused, and tired all the time?" God responds from a speech bubble in the sky, telling him: "In a past life you were a hot billionaire genius who had non-stop sex. This lifetime is a punishment for that one."
In the third panel, the man reacts with a wide-open mouth as if screaming in existential horror at this revelation. But in the fourth panel, he is peacefully asleep, snoring away ("zzz..."). The answer, however absurd, was apparently satisfying enough to put his mind at ease and let him sleep.
The Humor
The comic plays on the human need for explanations. The man's suffering keeps him awake not because of the suffering itself but because he cannot understand why he is suffering. Once God provides an explanation -- even a completely absurd one involving reincarnation-based cosmic justice -- the man is immediately at peace and falls asleep. The joke suggests that humans do not actually need good answers to life's difficult questions; they just need any answer at all.
There is also a darkly funny irony in the specific explanation: the man's miserable current life is supposedly punishment for a past life that was perfect in every way. This parodies karma and reincarnation concepts by suggesting the universe operates on a petty system of balancing cosmic jealousy rather than moral justice.
References
The comic references concepts from Hinduism and Buddhism regarding karma and reincarnation -- the idea that one's current life circumstances are determined by actions in past lives. It satirizes this by replacing moral cause-and-effect with a simple ledger of "you had it too good, now you have it bad." The comic also touches on theodicy, the philosophical problem of explaining why a just God permits suffering.