Albert Barnes Commentary Numbers 18:6-7

"And I, behold, I have taken your brethren the Levites from among the children of Israel: to you they are a gift, given unto Jehovah, to do the service of the tent of meeting. And thou and thy sons with thee shall keep your priesthood for everything of the altar, and for that within the veil; and ye shall serve: I give you the priesthood as a service of gift: and the stranger that cometh nigh shall be put to death." — Numbers 18:6-7 (ASV)

The Lord instructs here the priests that the office which they fill, and the help which they enjoy, are gifts from Him, and are to be viewed as such.