When we add water to milk, they get mixed properly because:
Particles of water get in between particles of the milk.
Particles of matter have space between them.
Thus, when two matters are mixed, particles of one matter get into the spaces between the particles of another matter.
When we add water to milk, the particles of water enter the empty space between the particles of milk and inversely the particles of milk also enter the empty space between the particles of water. Thus, it gets mixed properly.