I always wondered why Ben removed the Omnitrix in the first place, I mean there was probably crime in Bellwood at some point, so why on earth would he remove the watch? And would you remove it if you had it? Because if I had it, that watch will never leave my wrist.

He initially retired...

He wanted to be a normal kid

@jay-10:

I think it was a way of finishing the series. But also to be a normal kid.

To be a normal kid is one reason. Me personally, think it was something messed up or tragic. For instance, probably thought no more alien invaders would arrive if there wasn't an omnitrix to come for. Or something else . In omniverse, bens favored alien form, feedback, was literally torn out of the omnitrix and destroyed in front of him. And out of rage, ben used the omnitrix power to destroy the alien responsible. I believe such an event may have scared and traumatized the hero out of him, causing him to want the omnitrix removed...

@jay-10: Do you mean the start of alien force? or did he remove it again in alien force

@w0nd: I mean the start of Alien Force, it was removed once by Azmuth when he was getting attacked by Vilgax and on last time so Vilgax wouldn't hurt Gwen and Kevin, but got the Omnitrix destroyed so Vilgax wouldn't use it