Eager and friendly, Bumblebee nevertheless developed a solitary nature after the injury that left him mostly mute.  It is a wlcome surprise to discover on meeting Sam that he can actually communicate with humans through their music.  Of course, now that he's made friends on earth, he's got to fight to keep them alive long enough to actually enjoy the friendship.  If the decepticons want to harm the humans under his protection, they'll have to go through him - and his plasma cannon - first.