It is in Mercedes and Lewis' interests that Valtteri is in better head space for 2019. Lewis knows that over a season, Valtteri will only better him two or three times. But he needs to rely on Bottas being close to him to ward off the Ferraris. Mercedes need Bottas there to help secure the WCC by being between Lewis and the Ferraris and any other competitive cars...by challenging for wins when he can and picking up the pieces if Lewis bins it, is hit by another car or has a mechanical problem. Bottas is excellent at all of these things when he's in good head space. He's even capable of beating Lewis on his day.