Here's why:

* Diethylamine (Et₂NH) is a weak base. It has a lone pair of electrons on the nitrogen atom, which can accept a proton (H+).

* Hydrochloric acid (HCl) is a strong acid. It readily donates a proton.

When they react, the proton from HCl is transferred to the nitrogen atom of diethylamine, forming the diethylammonium ion (Et₂NH₂+) and chloride ion (Cl-):

Et₂NH + HCl → Et₂NH₂+ + Cl-

This reaction results in the formation of a salt, diethylammonium chloride (Et₂NH₂Cl).