Because water overcomes the force of cohesion in the salt, and thus separates its particles. The capillary attraction between the salt and the water is greater than the cohesion between the particles of salt; hence, the salt is dissolved by the water.
Gravitation.
When a ball is thrown into the air, why does it fall back to the earth?
Because the earth attracts the ball.
Why do rain and snow fall to the earth?
Because the earth attracts them.
What is this kind of attraction called?
It is called attraction of Gravitation.
Does the attraction of gravitation exist in all things?
Yes; every substance attracts every other substance.