To Frederick Jackson Turner, the settlement of the West was a good thing and a bad thing. The process of settling it was good, but the fact that the West became settled was bad for the United States.
To Turner, the existence of a frontier was basic to the American character. Turner believed that Americans' basic character was formed by the fact that they had always been able to go out and "conquer" some new area of land. This allowed Americans to be independent and and allowed democracy to flourish. It allowed them to be ingenious and resourceful, which helped the US economy.
This meant that the "closing" of the West was bad news. If there was no longer a frontier, how would Americans challenge themselves? How would they maintain their independent and resourceful spirits? These were serious problems in Turner's mind.