According to the Molecular Orbital Theory, what makes a compound aromatic?

For example, benzene has 6 $\pi$ electrons. Which means the p orbitals will be occupied, and only the bonding molecular orbitals. But 1,3-cyclopentadiene has 4 $\pi$ electrons, which means 2 of the 3 molecular bonding orbitals will be occupied. Why does this make it non-aromatic?