Because 789

Q: Why was 6 afraid of 7?

A: Because 789

The most common answer to the question, because 789. Now on paper this seems ludicrous, you might be thinking to yourself: why would six be afraid of three numbers? Do those numbers mean something in a Dan Brown-esque sort of way? Is 789 the area code of a dangerous neighborhood?! WHY IN GOD'S FUCKING NAME IS THE ANSWER 789?!?!?!

I commend you for getting so worked up about this.

The real reason this joke works is that it's a play on words. 789, when written out means seven eight nine. Eight sounds identical to the word ate. Ate means to consume, as in the past tense to eat. Therefore we look and see that seven ATE nine, as in consumed the number following it by two. We then assume that these numbers are personified, meaning that seven represents a human in our brains, who has the audacity and strict bodily mass to force nine into a submissive role and consume him. Thus we picture in our heads the horrifying image of a ripped and masculine seven destroying a meek nine much as a lion destroys a gazelle.

It seems clear to us now that six would be frightened of seven.

No one knows if 1, 2, 3, 4, 5, 8, or 10 are frightened. We assume 8 is just angry because his name was used in the joke.