As a yami, Yami knew he would always be prone to giving into his dark side. Being locked inside a Millenium Item for five thousand years does things to a person. Now everything was different. He wasn't a pharaoh in this life and didn't have the weight of a people on his shoulders. He had his friends and he had Duel Monsters, but most importantly, he had Yugi.
Yugi was the light of his life, both figuratively and literally. As his hikari, Yugi was his light side. He stopped him from making bad decisions and suffering their consequences. As for the not so literal, Yugi had saved him: from the Millenium Puzzle, from the darkness, from himself. Yami knew that he might never be able to reciprocate, but he would spend the rest of this life trying. Yugi was the love of Yami's existence. In his first life as a pharaoh, he'd always felt that something was missing. In this life, he'd discovered that it was Yugi. Yugi was good to him, good for him. He could only hope that his hikari felt the same way about him.