Jesus was God because he was the Son of God. Like begets like. The child of a shark is a shark. The child of a dog is a dog. And the child of God is God. Since God was his father, he partook of the divine nature.
But he also had a human parent, Mary. From her Jesus got a human nature. I'm not sure how the divine and human coexisted in him, but some things you just have to take on faith.
One thing we should never have any nonsense about it being easy for him because he was really God. From his birth, when there was no place for him but a stable in the world he made, to his death, during which the Father with whom he'd had eternal communion turned his back on the Son he'd always loved, divinity made it harder, not easier,