Why did God create Man

Throughout the old and new testaments the same reoccurring text appears, "they shall be My people, and I will be their God." We were created to be His people, but we rejected Him.

Ezekiel 16:50 And they were haughty, and committed abomination before me: therefore I took them away as I saw good.

What does it mean, I will be their God? ------ No one knows ......We've never been His people