As Solo mentions, a lot of the bad blood goes back to 93 when Michael signed to race at McLaren. That was HUGE news for F1, and then Michael proceeded to have less than a stellar performance. He made a lot of comments about not having to move to Europe to be successful. His wife was a prima donna, he thought he was super special and then the lack of results. He flew back and forth for races, meaning he never built up capital with the team, teammate, journalists, TV, etc.
Just look at what drivers like Lauda, Schummi, Hamilton have done to lift the team. They spend time at HQ with the 'boys', engineers, pit crews, etc, to get everyone playing above their skills. So much to be said for that just like it is here stateside in _______ (name your sport).
Senna is LOVED throughout the racing world (unless you were a Prost fan) and is a GOD in Brazil. He was the son of wealth, but raced like a common man with a big chip on his shoulder. What he could do in a car was other worldly. Dare I say, you could put him in the Haas and he would finish no lower than 5th in the Championship. In the rain, only Max is even close.