If one side has one, the other side has to get one. Once launched, they cannot be recalled. When they land, they screw up everything forever.
or
Because it would fall over if it lifted the other one.
or
To hide in a bag of M&M's.
or
It takes them a long time to swallow their pride.
or
They turn from notating nuclear spins to notating unclear puns.
or
Because pepper makes them sneeze
or