Raoul spends much of his time in the novel being sad and angry. Raoul's heart was broken when his fiancé, La Valliere left him for King Louis XIV. Why can't Raoul get over La Valliere? Does it seems as though he doesn't want to get over La Valliere? Perhaps he prefers his melancholic existence. For whatever reason, Raoul can't recover from the fatal blow dealt him by his lover. He decides to head bravely into battle, reasoning that it is better to risk his life in war than remain under the spell of La Valliere. Why does he prefer to die rather than stop loving her? Well, that is a question that you're going to have to answer for yourself.
We think that Raoul's hang-ups (both with regard to La Valliere and his decision to enlist in M. de Beaufort's army), might have something to do with his passionate personality. Raoul dedicates himself to people and causes he believes in. Once it becomes clear that the love of his life no longer loves him, (and remember that D'Artagnan himself urged Raoul to move on), Raoul needs something else to which he can cling. He might see his role in the war as his only means of getting over his heartbreak. According to M. de Beaufort, Raoul fought and died bravely, never once running from the many challenges he faced.