Question

Resonance is possible only in aromatic compounds. Why?

Answer 1

You need to understand the basic reason for the aromatic nature of materials. A molecule is said to be aromatic only if:

A ring in it is planar,

Continuous delocalisation is possible throughout the concerned cyclic ring, and

This delocalisation stabilises the ring.

Now, for continuous delocalisation to be possible, the π orbitals of all the elements in the ring need to be parallel to each other ( They need to overlap sideways for formation of π bonds).

In your example, consider the carbons in the ring. All of their π orbitals are parallel to each other and PERPENDICULAR to the plane of paper. On the other hand, the imine nitrogen is sp2 hybrid. The lone pair of electrons is in an sp2 hybrid orbital, i.e, IN the plane of the paper. And mind you, it's fixed (can't rotate without distorting the geometrical shape of the ring, that is, which would be too much to expect from the poor guy.)