Asante Samuel says playing for Bill Belichick and the Patriots was NOT fun ... and he tells TMZ Sports he believes the reason Tom Brady bolted was 'cause he just couldn't take it anymore.
"He probably just got fed up with it at the end of the day," the former New England cornerback says.
Samuel played with Brady and Belichick from 2003 to 2007 ... and he had a TON of success -- but he tells us he had ZERO fun along the way.