Why aluminium oxide is amphoteric?

Aluminium is amphoteric because it can form both an acid and a base e.g.

Reaction as an acid : Al2O3 + 3 H2O + 6 H3O+(aq) ? 2 [Al(H2O)6]3+(aq)
Reaction as a base : Al2O3 + 3 H2O + 2 OH-(aq) ? 2 [Al(OH)4]-(aq)