He was writing to a teenage girl and tbh, his reply was right, but in a way that most people without depression could understand. And the sun WILL come out, unfortunately we never know when.
Accepting is different to giving in. Acceptance could potentially give someone their permission to wallow in their MH, and trying harder could potentially make it impossible for that person to understand that they need to look after themselves.
For me, I've accepted I have depression and anxiety, with all that goes with that, so I do need to take care of myself (take my medication, eat healthily, get enough sleep, avoid triggers and devise coping mechanisms). That's not giving in, and I always work hard to have the best life I can have, but I do that knowing my limitations. Usually