There is one and only one reason Trump/US wants Greenland.
There’s more than one. To me, the most plausible one is that Putin has played on his insecurities. He probably told Trump that a president / king / emperor is remembered when they expand their territory, otherwise they’re forgotten. So, off Trump goes, trying to cement his legacy as a great president by expanding US territory. By doing that, he plays right into Putin’s hands by destabilizing the world.
“Evil” people have motivations too. There’s no overthinking needed. He’s frequently played by Putin, and this makes the most sense.