It's clear from the first time we see Sergius and Raina together that they are far more in love with the idea of being in love than they are actually in love with each other. They are both of the same social class, and it is probably a fond wish of both of their parents that they marry each other. They are suitable for each other ; Raina is beautiful and young and the daughter of a prominent Bulgarian, and Sergius is a rich and gallant soldier. By romantic notions the two should be perfect for each other. But their meeting is strained and syrupy, when they spout only platitudes toward each other. Raina makes the riculous protestations of her virtue to him, while she is actively deceiving him
I think we two have found the
higher love. When I think of you, I feel that I could never do a
base deed, or think an ignoble thought. (Act II)
And the minute Raina is out of the room Sergius is making advances toward Louka. It is evident that both Raina and Sergius have reasons to keep up the pretense of loving each other, although neither of them seems to feel it. Sergius has flirted with Louka before, and Raina had made quite a connection with Bluntschli when he was hiding in her bedroom. But the pretense of the "higher love" (Act 2), which is against both of their own feelings, must be kept up -- for appearance's sake. But Sergius even confesses to Louka that it is "very fatiguing" to do so; it is a duty, and not a feeling. Therefore, if Raina and Sergius had been married it would have been a loveless marriage.
Also, Sergius is a whole-hearted soldier, and Raina has found that her sympathies are far more pacifist. She has discovered these feelings while talking to Bluntschli who, though a soldier himself, knows the folly of war.
In the end, the conniving Louka and Sergius are much better suited to each other, and the more thoughtful and pacifist Blunschli and Raina belong together. Or at least this is what Shaw seems to imply!