Revealing himself to be Yoda, the Jedi Master began Luke's training in earnest. Telekinetic challenges, challenges of the will and body, more challenges than Luke had ever faced before helped mold the reckless youth into a Jedi.
Still, Luke failed to "unlearn" his preconceptions.
When asked to raise his sunken starfighter from the Dagobah swamps with the power of his mind alone, he responded he would try.
"No," scolded Yoda. "Do, or do not. There is no try."
Luke did not believe the Force could lift such a massive object.
He was proven wrong when Yoda telekinetically lifted the X-wing fighter and placed it on dry land.
Again, Luke was incredulous -- he did not believe; that is why he failed.