The most obvious answer is the sheer magnitude of his narcissism. Trump is very unpopular among young adults. Most of them believe that Trump is immoral and sexist. Many people also think that Trump is a racist. Trump made some remarks in his public appearances that were interpreted as racist remarks.
Many people also consider Trump to be arrogant, and they think that he has a massive ego. Trump has also criticized and humiliated people in public. Obama fans dislike Trump. Trump criticized Obama many times during his presidential elections, and he claimed that Obama was lying about his place of birth.