Impeaching a president is the single greatest condemnation that a nation can give a president. How is that not a very serious thing?

Because of the partisan joke Schiffty made of the "investigation"
If they were serious they would have appointed a special prosecutor to eliminate calls of partisionship. But Mueller report was such a disappointment they could not risk that again.

It is serious when it is undertaken for crimes committed. To have boiling, seething, uncontrolled hatred for someone is not a "high crime or misdemeanor". That more or less falls under the category of I want it, I demand it and I'll abuse my own power and authority to obtain it. It seems to be a derangement that the majority of people can't grasp nor tolerate. Those of us not blinded by an outrageous lack of sensibility and maturity have our work cut out for us in cleaning up the mess made by the House of Representatives.

Trump has Narcissistic Personality Disorder. He's a psychopath.
As part of the traits which define these conditions,
he does not respect the rules of government, constitution or authority.
He does what suits him and then engages in lies and cover-ups to attempt to avoid being caught.

In the case of this impeachment, his Tweets prove that he's been very riled by it -
but only in the sense that he likes to believe it's an unjust witchhunt.
He doesn't believe the process has any real teeth because it will lose the vote in the Republican-dominated Senate.
And, unfortunately, he is right about the votes.