9 slapped 8.

8 asked, "Why did you slap me"?

9 replied, "You are inferior to me, so I slapped you."

Upon hearing this, 8 went to 7 and slapped 7 harder.

When asked by 7 why, 8 gave the same reply that 9 gave.

"You are inferior to me".

Likewise,

7 slapped 6.
6 slapped 5.
5 slapped 4.
4 slapped 3.
3 slapped 2.
2 slapped 1.

Now 1 has 0 as its inferior.

But, instead of slapping—

1 welcomed 0 with love. Allowed 0 to stand next to it.

The duo became 10.

Tables turned and it is now number 9's turn to get slapped.

Advice:

Choose your relations wisely.
A good friend is as worthy as 1000 good books.
A bad friend is as harmful as 1000 enemies.
Even a mighty snake dies if a group of ants attack. Never be proud of your position. Stay grounded.