Kasper's explanation to Katrine
Spoiler...
In the 3rd season after katrine had been going through a bit of an emotional roller coaster with Kasper and eventually snapped, I thought the way Kasper explained to Katrine why he had fallen out of love with her was so well written.
Through tears he explained to her that he had gradually lost his sexual feelings for her after she became a mother but he couldn't imagine wanting anyone else to be the mother of his children. His emotions were so genuine and honest, it was such a perfect scene. Her smile after hearing this was all that was needed to convey her feelings. This was exactly what Katrine needed to hear, she could finally let him go.
Such good writing.