Why is 3D stress tensor acting only on three surfaces?

I'm trying to learn about the stress tensor (in 3D)

The tensors are said to have directions (the first subindex $i$ in $\sigma_{ij}$) and specify the surface upon which they act (the second subindex $j$ in $\sigma_{ij}$).

What confuses me is why is it defined only to act on three surfaces, even when the cube has six surfaces?