It results in the unshared electron pair on nitrogen
atom to be in conjugation with the benzene ring and thus making
it less available for protonation If you write different resonating
structures of aniline,  you will find that aniline is a resonance
hybrid of the following five structures On the other hand, anilinium ion obtained by accepting a proton
can have only two resonating structures (kekule) We know that greater the number of resonating structures, greater
is the stability