For once, people have confidence that the government is actually going to get smaller. There are going to be fewer regulations than ever before. There are not going to be problem with the government shutting down businesses that they don't like. The stocks are going to soar more than ever before.
The Dow hitting 20,000, along with new economic reforms and tax cuts means that we could be at the beginning of an economic boom. The American economy has been depressed for years. Real GDP growth has been the slowest since the end of WWII. The Dow hitting 20,000 shows that investors feel more optimism.
No, the all time high for the DOW is not necessarily the start of an economic boom. Confidence is high right now because many people believe Trump is going to make miraculous changes in the country. However, as reality sets in and people realize there is no great magic coming and possibly even some problems ahead, the DOW will drop down again.
I think the DOW reaching such a high is a false positive. If it turns out to be a boom then it has absolutely nothing to do with Trump being elected, but all to do with the past eight years of Obama policies that brought us back from a recession. The current president can only take credit for things at least after the first four years of his presidency.