I'm sure someone will call me out on being wrong because someone always knows more than me in WW2 topics but what the hell I'll take a stab:
Hitler believed that the Soviet Union was practically a failed state and there wasn't a whole lot of evidence contrary to that belief going into Barbarossa. They purged much of their military leadership and waged a shockingly embarrassing war against the Finnish. I think Hitler was quoted as saying (and I'm paraphrasing) that if you kick the door down, the whole rotten structure will collapse.
The Soviet Union also had vast grain, mineral, metal, and oil resources that the Germans needed to sustain the war effort and wouldn't be threatened by a naval blockade.
I've seen other assertions that Barbarossa was, in essence, a grand counter offensive because there's belief that the Soviets would have attacked Germany eventually anyway.
I'm not sure if the Germans underestimated the Soviet's ability to move their heavy manufacturing capability east or the lengths they would go to with their "scorched earth" policy. I think the general consensus seems to be that as a result of their ineffective supply lines partially as a result of the Soviet scorched earth policy, even if Barbarossa happened on time in May 1941, that the end result would be the same. Moscow and just beyond was just too damn far for their logistics.