Mostly because Geralt and Kratos are pretty similar.
-Both are big buff warriors.
-Both are angry boys
-Both are snarky boys
-Both Geralt and Kratos usually communicate in grunts, growls and 'hmms' most of the time. (They consider the other their best friend)
-Jaskier and Mimir LOVE to sing duets which Atreus enjoys immensely but both Geralt and Kratos DO NOT. Eventually leading to both saying "Quiet bard/head!" Several times.