The definition of S1 depends on Vx[i1, j1] and the definition of Vx[i1, j1] depends on S.

A simplified example would be:

a = b
b = a + 2

This is how the evaluator thinks about this.

a = b
(* Every instance of the symbol a will now be replaced by the symbol b *)

b = a + 2
(* Oh look! It's the symbol a. Better replace it with b *)

b = b + 2
( * All instances of b should be replaced with b + 2 *)

(* Oh look! it's the symbol b. Better replace it with b+2 *)
b = b + 2 + 2

(* Oh look! it's the symbol b. Better replace it with b+2 *)
b = b + 2 + 2 + 2

(* Oh look! it's the symbol b. Better replace it with b+2 *)
b = b + 2 + 2 + 2 + 2

.....