ולקחת מראשית כל פרי האדמה… (כי תבוא כו:ב)

The nature of a person is that he prefers the first fruits that are picked off the fields. When a person takes these fruits and he separates them for Hashem, it breaks his תאות האכילה – eating temptations. Noble people, such as kings and lords brought the Bikurim themselves, standing humbly before the Kohen, thus lowering their levels of greatness and using the trait of being humble before holiness.