Here's why:

* Ionic Compound: Ammonium sulfide is an ionic compound, meaning it's formed by the electrostatic attraction between positively charged ammonium ions (NH₄⁺) and negatively charged sulfide ions (S²⁻).

* Polar Solvent: Water is a polar solvent, meaning it has a positive and negative end due to the uneven distribution of electrons. This polarity allows water molecules to surround and interact with the ions, breaking apart the ionic compound and dissolving it.

Note: Ammonium sulfide is a highly unstable compound and decomposes readily in solution, releasing hydrogen sulfide gas (H₂S) which has a strong, rotten egg smell. This is why it's often used in laboratories as a source of hydrogen sulfide.