Trump also wanted to rename the Gulf of Mexico to Gulf of America. This is all about vanity, not about minerals, economy, security etc.
In his press conference Trump mentioned the world map multiple times. Trump may well be forming his annexation ambitions based on what the map would look like after the annexation. Well, the world map looks very different based on what kind of projection you use. In the Mercator projection Greenland looks big. Bigger than the US, in fact. This may be what is attracting Trump, as it looks like a lot of real estate. Tobler hyperelliptical projection on the other hand makes Greenland look small. Someone should show Trump this projection and it may very well cause him to lose interest, and we can sidestep a dispute with Denmark and Europe.