why did the reoncarnated majin buu, uub need training from Goku if he almost killed Goku?

Goku not training uub plus like they train together uub the only person beside vegeta that can give Goku a good fight

He didn't almost die he just wasn't in his super saiyan form.

Uub needed help just unlocking his full potential and using his destructive power at will and controlling it at his whim

because he is a good guy now and plus clearly is because Goku is the pure-hearted saiyan

because his power did not get use to the new body yet