Why NH3 is basic in nature?

Answer 1

Because ammonia will, weakly, accept protons in solution.

Answer 2

NH3 is basic in nature because it acts as a Brønsted-Lowry base by accepting a proton (H+) from water, forming the ammonium ion (NH4+) and hydroxide ion (OH-) in solution. The lone pair of electrons on the nitrogen atom in NH3 can readily accept a proton, leading to the formation of OH- ions, which increase the pH of the solution, making it basic.