Why does Sullivan Balliou tell his wife not to mourn him when he is dead?

He felt as if his love for her was endless. Yet, he also felt that the cause he was serving was right and he was willing to put everything he had in to support it. As because his love was endless for her, he didn't want her to be pained and as he knew he would probably die in the following battle, he wrote the famous letter telling her that it was going to be okay and that she didn't need to mourn his death.