I am, the Lord your God - The frequent repetition of this formula in these parts of the Law may be intended to keep the Israelites in mind of their covenant with Yahweh in connection with the common affairs of life, in which they might be tempted to look at legal restrictions in a mere secular light.

Speak unto the children of Israel,.... To the heads of their tribes, that they might deliver to them the following laws; or Moses is bid to publish them among them, either by word of mouth, or by writing, or both:
and say unto them, I am the Lord your God; with which they were to be introduced; showing the right he had to enact and enjoin such laws, since he was Jehovah, the Being of beings, and from whom they received their beings; their sovereign Lord and King, who had a right to rule over them, and command what he pleased; and also the obligation they lay under to him to regard them, and yield a cheerful obedience to them, since he was their God, not only that had made them, but had redeemed them out of Egypt; and who had made a covenant with them, and had taken special care of them, and had bestowed many wonderful favours on them; and for this purpose is this phrase often used in this chapter, and very frequently in the next. See Leviticus 18:2.

UNLAWFUL MARRIAGES. (Leviticus. 18:1-30)
I am the Lord your God--This renewed mention of the divine sovereignty over the Israelites was intended to bear particularly on some laws that were widely different from the social customs that obtained both in Egypt and Canaan; for the enormities, which the laws enumerated in this chapter were intended to put down, were freely practised or publicly sanctioned in both of those countries; and, indeed, the extermination of the ancient Canaanites is described as owing to the abominations with which they had polluted the land.

Your God - Your sovereign, and lawgiver. This is often repeated because the things here forbidden were practised and allowed by the gentiles, to whose custom he opposes divine authority and their obligation to obey his commands.