Answer
A Bronsted base needs a lone pair to form the bond between it and the proton ($H^+$).
Work Step by Step
The definition of a Bronsted base is that, it has the ability of receiving a proton.
To form a bond between this base and that proton, it is necessary to have a pair of electrons.
Since the proton doesn't have any pair, it has to come from the Bronsted base.
