This is the exact expression he gave to Thor during the elevator scene: the surprise at hearing something he so desperately needed and the sadness because it’s too late. In this scene, he recognizes that Odin is dying, and in the elevator scene, Thor tells him that he thought the world of him only in the past tense, so there’s acceptance in his gaze but also a wistfulness.
Tom Hiddleston said that Thor and Loki are “brothers and friends and allies” and “two halves of the same whole” and “born to be together“ and I think I might have actually died a little listening to him speak those words.