I just watched that clip from "Brief Encounter" in your video, with Leslie Howard. The film is about a schmaltzy puppy-love non-romance between two utterly boring and shallow, uninteresting people, in dull marriages matching their personalities. I believe the music suited the story precisely because it matches their emotional and inner lives when they discover this spark of passion within a life utterly devoid of anything resembling passion. It turns out what they feel for each other is pure illusion, the rebellion of stifled and arrested emotions against their own self-accepted humdrum passionless conformity and ordinariness. People like that have very limited horizons in every respect, including their sense of love or passion. So the music David Lean uses to convey this inner experience is Rachmaninovian sentimentality. If you look at the "love story," it's very clearly a very silly flirtation between two goofy and shallow people. The music was chosen to match them.