Washington (CNN) On Thursday morning at 6:19 a.m. ET, the President of the United States tweeted this : "Crazy Joe Biden is trying to act like a tough guy. Actually, he is weak, both mentally and physically, and yet he threatens me, for the second time, with physical assault. He doesn't know me, but he would go down fast and hard, crying all the way. Don't threaten people Joe!"

That tweet by Donald Trump came in response to comments former Vice President Joe Biden made in a speech in Florida on Tuesday in which he said of Trump: "They asked me would I like to debate this gentleman, and I said no. I said, 'If we were in high school, I'd take him behind the gym and beat the hell out of him."

On one level this is all dumb. Two septuagenarians debating who is tougher and beating their chests is not exactly the sort of stuff we expect (and we should expect) from our leaders. (Sidebar: Biden is far from blameless in this. His comments were pure machismo and ill-advised. And, yes, Biden is suggesting assault on the President of the United States.)

But, that Trump decided to respond to Biden -- and that he responded the way he did -- shouldn't be dismissed. It provides -- as almost all of Trump's tweets do -- a window into who he is, how he sees the world and what being the President of the United States actually means to him.

Here are three thoughts on what Trump's tweet tells us about him.

1. Being "presidential" is not a thing for Trump

What most presidents would do in a situation in which a private citizen -- albeit one who used to be the vice president of the United States and is actively considering a future run for president -- attacked them is to ignore it. What possible gain, after all, does a president of the United States, just 14 months into his first term, gain by engaging directly with a political opponent? It makes him look like just another politician rather than the President.

Trump doesn't care what other presidents would do. Or, he cares in the sense that it likely spurs him to do the opposite. He has made a career out of zigging when everyone else zags. So, if most presidents would ignore Biden, Trump goes after him -- and punches back 10 times harder than he got punched.

Trump has zero concern for whether doing so is "presidential."

2. Trump sees himself as a street fighter

At his core, Donald Trump is a New York kid who grew up feeling like he was always on the outside looking in, having to scrap his way to get into the places he wanted. (That image doesn't totally jibe with the reality of growing up as the son of a wealthy real estate developer, but I digress.)

For Trump, combat -- more the rhetorical kind than the physical kind -- is what he knows best. He likes having an opponent, a foil, someone he can use as a punching bag to score points with the people who support him. In that, Biden is perfect. A Democrat. Who was part and parcel of the Obama administration. Who screwed everything up in the first place. And on and on and on.

Trump, in his mind's eye, is also a tough guy -- a big, imposing figure who isn't just some schmuck in a suit. He peppers virtually every speech he gives with tales of his strength, his toughness, his manliness. Other people may make idle threats, but not me, Trump is forever suggesting. You step to me and I'll knock your block off -- or whatever the kids say nowadays.

3. Trump is very, very frustrated

I wrote on Wednesday about the increasing pressure crushing down on Trump -- from a series of civil lawsuits brought by women alleging affairs or unwanted groping, from special counsel Bob Mueller's probe into Russian election meddling, from the lack of close confidantes left to him in the White House.

When you have that much pressure on you, you need to find ways to blow off steam -- or risk exploding. Twitter has long been Trump's pressure release valve, the way he feels he can communicate in real time with the people who truly "get" him -- his followers.

Tweets like the one proposing a fight with Biden are indicative of a man who feels trapped by his office and hemmed in by a series of issues that he has little to no control over.