Stack push operation is O(1) because: 1) Only requires adding element at known top position, 2) No need to traverse or reorganize existing elements, 3) Direct access to insertion point via top pointer, 4) Independent of stack size, 5) Amortized constant time even with array resizing, 6) No search or comparison operations needed, 7) Optimal for rapid data insertion, 8) One of the key benefits of stack data structure.