In positive multiples of 9 (e.g. 18, 72, 909) why do the sum of the digits (no matter what order or combination) make 9 or another multiple of 9?

9 = 9 = 9

18 -> 1+8 = 9

27 -> 2+7 = 9

909 -> 9+0+9 = 18 (1+8=9) or... 90+9=99 (9+9=18, 1+8=9)

So no matter how large the multiple is, you can deduce it by summing its digits until you eventually reach 9.

Also, is there a way to prove this for every multiple of 9?