The priests are holy to their God

Leviticus 21:1-24
A priest must not make himself ceremonially unclean for any of his people who die, except for a close relative, such as his mother or father, his son or daughter, his brother, or an unmarried sister who is dependent on him since she has no husband.
They must not marry women defiled by prostitution or divorced from their husbands, because priests are holy to their God. 
No descendant of Aaron the priest who has any defect is to come near to present the food offerings to the Lord.
Why?
A priest himself have to be ceremonially clean, even marriage and his descendant who has any defect is not allow come near the altar to present offering to the Lord, but he may eat the most holy food of his God.
God is holy and he loves the descendant of the priest.
Because of their position and duties the priests are subject to particularly stringent regulations on ritual purity. Anything which makes them 'unclean' means they may touch anything in God's holy sanctuary. The rules for the high priest are even stricter.
How?
To be a minister of the Lord as me, I should keep obey the command and decrees of the Lord, because he is Holy.