Logic based answer: Giving Keith the two years of space was very much neccessary to his emotional development. He needed that time to both bond with his mother and do some introspective self-discovery. Before, Keith was at a place where 1) shouldering the burden of leadership and piloting the Black Lion was detrimental to his mental health, and 2) he believed his mother had never wanted him, and most of his abandonment issues stemmed from that. Allowing him the time and space to come to terms with things at his own pace, allowing him literal years of growth,, that’s invaluable. There’s so much he could have learned from his mother in that time–about being in the BOM, about being a leader, about simply being part galra.
He was also just…given this wonderful, cozy little break for a bit. You have to remember, Keith never had a childhood. This is the universe giving Keith a break, this is Keith finally getting back that stolen time. Finally, he can have this happy little dosmetic life–complete with his mom and a dog. It’s probably the sort of thing he dreamed about as a kid. He was given 2 whole years to get to know the mom that was taken away from him, was able to come to terms with her decisions and his own. It’s a chance for self-reflection, but more than that, it’s time to heal.
My gay heart: All of that is immeasurably important. But also, Keith has now come back a changed person, more mature–and it really shows. It’s something that “Shiro” himself takes notice of. Keith isn’t the same person he was all those years ago at the garrison. Having been given time and space to think and reflect on his past bonds–including his relationship with Shiro–he was able to come to terms with a truth that still eluded him in the BOM.
He was able to face Shiro and say, “I love you.” Also, they’re closer in age now. At 21 and 25, a romantic interest there is even more likely. Nevermind that Keith has had two years to ruminate on the potential doomed future of Shiro, and the whole idea that “absence makes the heart grow fonder.” All of this no doubt helped him come to terms with his feelings.