Is one the best things in the world. Even after his diagnosis Steve kept coming in because it was fun environment. Over time his interaction in the room was slowed down, he still came up with the idea of SpongeBob and Patrick coz playing as MM & BB.
Which was a gift that kept on giving.

Steve realized that he missed working on SB and decided to rejoin day to day of the production. Not that he was in it up to his elbows like the 1st 3 seasons. He was mostly interested in the writing and animatic meetings. Which are truly fun.
Getting to bounce ideas around in room full funny people

He and Derek would work into the night to try and plus up boards they felt needed help.
A sudden and substantial cash wind fall, opened the door for him to spend more time with his family and to decide what else he wanted to do. 5 seasons later after Steve came back to work on the 2nd movie

I think most of us, if we won a lottery would need to do some serious soul searching over whether or not to continue working 9to5. Even if you love the work.
Steve put in 3 seasons working long hours, away from his family. He did not just give notes on pitched storyboards.

Cont. Naturally Nickelodeon wanted to continue, and after some thought, Steve agreed and put Paul in charge for season 4.

This is all just my opinion from where I sat. Steve worked very hard on the first three seasons, and when he received his first check for his % of the merchandising money ( It was a much bigger check than most could ever expect) he realized he could easily step away from the show.