Andrei is bitter, to the point that he even thinks his father is making fun of him not being in a winning battle. Though we don’t get his thoughts I think his attitude is a combination of feeling guilty about his wife dying, nervousness at raising this small child, disillusionment at having been in a battle that went badly, and a lack of direction. And he never really has direction until he falls in love.
Battle of Eylau is important to be mentioned here because it shows a weakness in Napoléon’s Grande Armée no matter how uncertain that may be. There might be hope yet at a time when Pierre and Andrei seem to have lost all hope in life, though at least Pierre has something to reevaluate his life with.