why do u think he took it off for three years?
cause grandpa knew this woulda happened, and told ten year old ben, who was then disgusted with the idea, so he took it off and hid it in his closet(wtf? most powerful weapon in the universe goes undiscovered for 3 years in some kids closet?) and now we have Ben Ten Alien Force.
but srsly thats how it starts out, he gets it out of his closet when some baddies come to town, and then it evolves into the green watch (to match his outfit of course) and he has ten new aliens but none of the old ones.
Anyways, Kevin isn't really trying to be good he just likes "hanging out" around Gwen, if u know what im sayin