I think "to feel comfortable" could be another acceptable translation. (Matěj felt comfortable among Žofie's friends.)
The moderators try to keep the translations as close to the original translation as possible, and as there is a word in Czech which means comfortable it probably won't be added. Comfortable is not a translation of dobře for as far as I know
But feeling "good" and feeling "comfortable" are not the same thing. As an example, I can feel "good"watching a movie, but may feel "uncomfortable" because the seats in the theater are hard.