Why can't NH_3_ donate a proton and act as a Bronsted-Lowry acid?

Answer 1

H
..
:N:H
..
H
NH3 has that single pair of unshared electrons (the lone pair on the left) which is just right for a bare proton (H^+) to move to. To DONATE a proton means it must break a N-H bond (in order to free up a bare proton) which takes energy.

Answer 2

That cleared it up a bunch. Thank you so much!