What is the reason that other characters help Matilde's search for Manech?

During World War I, in the area called “no man’s land” on the German-French lines, Manech was one of five French soldiers that was captured and pushed into the zone, where some or all were killed. As uncovering Manech’s fate becomes Mathilde’s obsession, she realizes that other soldiers’ families and loved ones also learned little or nothing. Mathilde also enlists the help of her family’s lawyer, Monsieur Rouviàre.
Not everyone wants to help her, and some want to put the episode behind them. Some of the relatives and friends empathize with Mathilde because of their mysteries and losses. Some are simply inspired by her devotion. Daniel Esperanza, the ex-sergeant, has become bitterly anti-war and, by the time she finds him, dying; he wants a clearer conscience. Aristide Pommier, who had served with Manech in the same regiment, helps her because the young men had been childhood friends. Célestin Poux, a former soldier who helped them men get food, is aware of his luck in surviving. Even some German soldiers’ relatives, such as Heidi Weiss, want to learn more about those dark days.