The NYT asked him about this a couple weeks ago. Here's an article with some excerpts from that [1]. Key parts:
> President Donald Trump revealed in a new interview with The New York Times that his quest for full “ownership” of Greenland is "psychologically important” to him.
> During a two-hour sit-down with multiple Times reporters on Jan. 7, Trump was questioned about why he won't just send more American troops to Greenland — which is legal under a Cold War–era agreement — if his goal is to fend off foreign threats. The president replied by saying that he won't feel comfortable unless he owns the island.
> "Why is ownership important here?" Times national security correspondent David E. Sanger asked.
> "Because that’s what I feel is psychologically needed for success," Trump, 79, replied. "I think that ownership gives you a thing that you can’t do, whether you’re talking about a lease or a treaty. Ownership gives you things and elements that you can’t get from just signing a document, that you can have a base."
> White House correspondent Katie Rogers — whom Trump recently called "ugly, both inside and out" for writing a story about his age — chimed in to ask, "Psychologically important to you or to the United States?"
> “Psychologically important for me," Trump answered. "Now, maybe another president would feel differently, but so far I’ve been right about everything."
[1] https://www.yahoo.com/news/articles/donald-trump-says-wants-...