There were two reasons why Johnny and Rab befriended Dove.  The first reason was because Dove was working for the British, and might reveal information about them that would be useful to the Rebels.  Paul Revere himself had instructed Johnny to watch Dove, "for if ever the British march out to attack us a colonel's horse boy might well know something is happening, in advance".  Indeed, it is from Dove that Johnny and Rab secure vital information on a planned attack by General Gage.

The other reason Johnny and Rab befriended Dove is because Johnny felt sorry for him.  Although Dove remained obnoxious and lazy as ever, Johnny's feelings toward him had changed.  When others mistreated Dove, "it hurt Johnny...he was like a man who owns a dull, mean dog.  He may punish it himself, but resents it if anyone else punishes it".  Unwittingly, Johnny had taken Dove under his wing.