Imagine your person A of your otp being much shorter than person B, but can only kiss when person B leans down to person A.
Now, it wasn't that he was a particularly short man (5'6" was a completely respectable height for a Regency gentleman thank you very much), it was more that the people in his life were rather tall.
Eric was built like a stocky lumberjack, the Director was of solid Aryan standing and young Ronald was a lanky, lithe creature. Even willowy Miss Sutcliff stood taller than him without her red heels and Alan resigned himself to being the littlest of their motley crew.
It meant Ronald used his shoulder as an elbow rest and Miss Sutcliff used his head as a chin rest (and the rest of him as a teddy-bear, she declared). It meant the Director remained seated at his desk when Alan submitted files.
It meant he had to grab Eric's (poor excuse of a) tie to coax him down so he could kiss him comfortably. They fit together incredibly well, he discovered early in their relationship, and realised he really didn't mind the height difference after all.