$$\ce{NH_3 + HCl -> NH_4Cl}$$
The electronic structure of ammonia is such so that the nitrogen atom has an outer shell with a lone pair of electrons.
When ammonia reacts with hydrochloric acid (above), the hydrogen from the $\ce{HCl}$ moves to the ammonia. However, it leaves its electrons behind because there is already a lone pair of 2 electrons on the $\ce{N}$ atom, allowing it to retain a $\mathrm{1s^2}$ shell.
Why doesn't the hydrogen take its electron over and nitrogen doesn't then "donate" an electron to the chlorine?