He's aware of the melting arctic due to climate change, he knows there will be competition, perhaps even war with Russia in the future or at least a scramble for drawing lines for resource rights. Greenland would provide a strategic location for further encircling Russia and is rich in minerals and access to undersea gas/oil fields and he loves, loves, loves getting more resources like a map-vidya-game player who thinks they alone are the key to success. It also can police and provide access to newly opened arctic shipping lanes.