Here's why:

* It accepts protons: Ammonia can accept a proton (H⁺) from water to form ammonium ions (NH₄⁺) and hydroxide ions (OH⁻). This reaction is an equilibrium, meaning it doesn't go to completion.

* Equilibrium favors the reactants: The equilibrium lies towards the reactants, meaning that most of the ammonia molecules remain unprotonated. This is why ammonia is considered a weak base.

In contrast, strong bases like sodium hydroxide (NaOH) completely dissociate in water, producing a high concentration of hydroxide ions.