Can anyone tell me why he left?

There was a great deal of speculation as to why Jonathan Taylor Thomas left the show when that whole situation played out... but the fact is that he left to continue his education. :-)

I don't mind that Johnathan Taylor Thomas left the show, the problem I have is that he didn't even have the nerve to come back and be on any of the last episodes, or even that back stage pass thing when the cast is reflecting back on the eight seasons. Home Improvement made him famous, at least he could have had a little nerve to show some respect and come back for that. But I guess I can't take away the fact that he was a great actor nonetheless, despite the departure.

The way I understand it Jonathan did want to be on the show for the end, but, he had family obligations that prevented him from doing so. As the story goes he was attending his GrandMother's (or Great Grand Mother's 95th Birthday celebration) but yes he's a great actor all-the-same

I had also heard that he and Tim got into it before the last show about him being there/not being there and that was kinda a "last straw".... like if he couldn't be there for longer he shouldn't be there at all so he went with not at all. <shruggs> I kinda feel bad for actors in that way... who do you please? Family or fans?

Well, whatever did or did not happen, Tim eventually admitted that he handled the entire situation poorly

I'm pretty sure he left to go to college at Harvard

All I know is that jtt left the show in order to start work on another film, but he had told Tim that he was leaving to attend Harvard, and Tim found out and that's where the whole fued started. The episode where Randy came home from Costa Rica, and the whole family kinda ignored him, that was totally real, because that was after they found out about him lying to them. And that explains all the tension in the show, and even on the show too.

that was the worst thing that ever happened to the show

I think that JTT want have his other movie and college. Tim just not want to work with him again.