Answer:
Cause: The electrostatic bonds among ions in a lattice only hold them together like two magnets with unlike poles facing each other. The water molecules can split them through the similar process with several molecules ganging up on them, the H+ parts of the molecules attracts the anion and O2- part; the cations. If this electrostatic attraction is big enough, the bonds between the solid's ions break (into separate ions). And so the solid dissolves. This is usually the case with most.
Why not covalent structures?: Sure, covalent solids have a low m.p and b.p for low intermolecular forces, the covalent bonds here are strong and cannot easily be 'pulled apart' by water molecules. They usually have stronger intermolecular forces than that of water.
But... if their intermolecular forces are weaker or similar, sure they can dissolve easily. Like any often-used alcohol (glacial).
