Dainsleif would've never imagined there would be a day where he'd want to settle. He was too focused on defeating the Abyss order to think of being with someone. Especially with his curse of immortality. That of course changed when he met you, someone who was also immortal.

Even as you separated, your goals being different, you were always at the back of his mind. The times you reunited were moments he cherished deeply, he didn't want them to end. Since Dainsleif was cursed to wander in Teyvat forever, he wanted to do it with you. Once he was done with his goals.