Why do we live in 3+1 dimensions?

In the context of string theory we argue that higher-dimensional Dp-branes unwind and evaporate so that we are left with D3-branes embedded in a (9+1)-dimensional bulk. One of these D3-branes plays the rôle of our Universe. Within this picture, the evaporation of the higher-dimensional Dp-branes provides the entropy of our Universe.