Well we do have proof he is a liar, don't we? I mean he must have known he couldn't fulfill a lot of his campaign promises, so we are sure he knowingly lied then. So why would anyone believe he isn't lying again?
And we know he hates losing which is why we have among other things his certainty that 3 million illegals voted which is why he didn't win the popular vote and more people at his inauguration than at Obama's. This last one is the most troubling, because he is disregarding facts, pictures of both events, and he is getting upset over something that is no big deal, of course Obama would get more people, Washington D.C. is Democratic, Hillary got 90% of the vote there, so that means 270,000 Washington D.C. residents didn't go to Trumps inauguration (that doesn't include the children of these people) so the people who did go to his inauguration most probably had to come from far away. Why would a person not see this very visible explanation for the lower attendance at Trump's inauguration, especially him?