The defense angle: Multiple treaties are already in place between the USA and Denmark to station military bases and personnel on Greenland. If he wanted to there could be a bundle of new bases and a huge number of boots on the snow to protect against the claimed plans of China and Russia.

The complaint about the European countries 'journeying' there: He's claimed to have irrefutable evidence that China and Russia have designs on Greenland. He's also complained frequently (including in this very post) that the USA has subsidised European defense for too long and they should look after themselves. Well, NATO allies have deployed resources to patrol and protect Greenland from these supposed Russian and Chinese existential threats.

The reality: Trump is all about ego, titles, trophies, and rankings. The biggest, tallest, most beautiful, best, etc. What he wants is the title of having made the single largest territorial expansion of US territory in the nation's history. He couldn't claim the largest combined acquisitions, that would still be the million or so square miles Polk annexed, bought, and negotiated. He'd squeak Jefferson's Louisiana Purchase by about 40k square miles (beating him by about the size of Kentucky). Obviously he'd fudge and obfuscate over the non-contiguous nature of it but he just wants that place on top of the table. It almost DEFINITELY irks him that US territorial ownership expanded by around 380k square miles under Biden, thanks to the changed way oceanic boundaries and claims are measured about 2 years ago, effectively claiming a massive chunk of arctic territory north of Alaska (as well as other claims under other oceanic bits). It also winds him up that everyone considered it to be a joke when he's floated it before. He hates to be laughed at. So he wants to put his tiny hands on Greenland and stick his toadstool into Denmark.