Shaw explains love and war by juxtaposing authentic love and realistic responses to war against false romance and false heroism.
In this play, Sergius represents both false romance and false heroism. Bluntschi, on the other hand, sees life realistically and represents genuine love and a pragmatic, rather than a heroic, approach to warfare.
Sergius pretends to be in love with Raina, the woman from his own class he is expected to love and marry. However, his true interest is in her maid, with whom he carries on a clandestine romance when Raina is not around. Sergius is also considered a military hero after winning a battle, but as Bluntschi points out, Sergius only won the battle because the other side was even more grossly incompetent than he was.
Bluntschi, in contrast to Sergius, doesn't pretend to be what he is not, and doesn't feel he has to play a cardboard heroic or romantic role. He deserts the battleground to save himself, which Shaw presents as a reasonable and positive response to his situation. He openly prefers chocolate to bullets. There is no false posturing in him. When he falls in love with Raina, it is based on genuine affection for her.
Shaw's play illustrates that is better to act in response to what our own hearts and minds tell us than to try to enact a false script of how life should be.