He said it himself....

Donald Trump's temper-tantrum tactics have been explained by the man himself. The frontrunner for the Republican presidential nomination admitted to his biographer that, "When I look at myself in the first grade and I look at myself now, I’m basically the same. The temperament is not that different."

https://theweek.com/speedreads/575962/donald-trump-tells-biographer-hes-same-now-first-grade