Question

Why interference of light is not taking place everywhere everytime?

Answer 1

Huygen's principale indeed says what you have mentioned, but be careful: each point of the wavefront behaves as a point like source of light. All these spherical waves do interphere together. That's the way the following wavefront is generated: it is the superposition of all the spherical wavefronts. In other words, the interpherence pattern related to this phenomenon doesn't allow dark regions. If you want to observe a typical interpherence pattern, with bright fringes and dark fringes, you need, for example a wall with two slits. When the wavefront impinges against the wall, all but two spherical waves that originate in each point of the wavefront are blocked by the wall. The two spherical waves that survive are those ones centered in correspondance of the slits. These two coherent spherical waves interphere and generate a typical pattern.

Answer 2

Explanation:

That is interference, resulting from each point on the irregular wall within the spot being at a different height and thus re-radiating the laser light with different phase. Each such point acts as a new source radiating in all directions. ... So: we do not need slits or gratings to produce interference