Ionic compounds usually dissociate in water because water is a polar molecule.
The O atom in water has a partial negative charge. The H atoms have a partial positive charge.
Electrostatic attractions hold the positive and negative ions together in the solid.
When the substance is placed in water, the water molecules pull the positive and negative ions apart from each other. The water molecules attract the ions from the surface of the solid. As a result, the ions move into the water.
The water molecules then surround the ion in an ordered fashion.
Several water molecules will surround a positive ion with their negative oxygens pointed at it.
The water molecules will surround the negative ion with their positive H atoms facing the ion.
This shell of water molecules surrounding the ions in solution stabilizes them and reduces the attractions of the positive and negative ions for each other.