There just aren't enough fics about how these two slowly fell in love became so close. Davos is an observant guy, it would probably take him about two days to realise that Stannis hates everybody around him. And while book!Davos is kinda more polite and more careful, younger Davos who has no idea how court works really wouldn't mince his words, not the way Stannis would encourage him to speak his mind.