People are saying it’s character development, it’s not.
Kratos was always a Dad of War, everything kicked off after he got tricked into killing his first family. They kept him tethered. It was hinted at his desire for family again in God of War 3 when he met Pandora.
That was the moment he actually softened up, and not 2018, but he still had a mission to do. When his mission was over he would have used that moment to get that feeling back by starting a new family.
But here’s the thing, Kratos is immortal, he’s said to be intentionally making his body look older for the sake of Loki. Loki isn’t immortal, eventually Kratos will lose him and when he does he will seek revenge and can revert back to his old look or another of his choosing if he pleases.