My hunch is that he wants something tied to him that he's confident will be carried on into perpetuity.
Trump's only major accomplishment from his first term is the overturn of Roe v Wade. The problem from a perpetuity perspective is that someday abortion may be legalized again across the country and it's not fully directly tied to him, it was a Supreme Court decision and others share the credit of overturning Roe v Wade by enabling SCOTUS to be able to make the decision. (In fact, Mitch McConnell probably deserves more credit than Trump.)
Trump knows that a lot of things change in politics, but that land gained is rarely returned. Add in his familiarity with real estate and I would guess that he'd like to acquire a major territory so that it can go down as his legacy. (And, of course, he'd likely try to figure out a way to name it or something associated with it after himself.)
That's my theory on his sudden interest in expanding the US.
“I met a traveller from an antique land,
Who said—“Two vast and trunkless legs of stone
Stand in the desert. . . . Near them, on the sand,
Half sunk a shattered visage lies, whose frown,
And wrinkled lip, and sneer of cold command,
Tell that its sculptor well those passions read
Which yet survive, stamped on these lifeless things,
The hand that mocked them, and the heart that fed;
And on the pedestal, these words appear:
My name is Ozymandias, King of Kings;
Look on my Works, ye Mighty, and despair!
Nothing beside remains. Round the decay
Of that colossal Wreck, boundless and bare
The lone and level sands stretch far away.”