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.