Hitler never trusted the professional army officers. His disdain came about because of the poor generalship in World War I that killed millions of German soldiers. He was always worried that the Army would attempt a coup de état and try to take over the government.
In 1934, as a result of the Night of Long Knives, he removed the SA, essentially a gang of thugs that had brought him to power, and killed their leader, Ernst Röhm. In exchange for removing the SA, the Army signed an oath of allegiance to Hitler.