He can't stop lying.
Sadly it is not just the number of lies and untruths coming from the president. Perhaps even worse is the way how this is used as part of of several strategies that is very common among the extreme right: among others an abuse of NLP (neuro linguistic programming) and an adoption of Putin's "deny distract and blame" strategy.
Part of this is to not just lie, but say the exact opposite of the truth and establish as the new normal. One example of this is the continuous use of "fake news" for reliable news sources while praising or being sympathetic towards low quality, hyperpartisan or actual fake news sources.
The worrying thing is not that some people use these strategies, what is really scary is how well it works with such a big part of the US population. I have no idea what to do about this, but I think if this trend continues (not just in the US but almost globally) we are in for really bad times, possible a larger military conflict, eventually.