Kylo has severe abandonment issues relating back to Han and Leia sending him away as a child to train with Luke. When he and Hux get together he has no experience of relationships. He's always fearful that Hux will leave him and needs constant reassurance. Hux knows he should find it annoying but he's never experienced someone needing him so badly and finds it really hot when Kylo gets all desperate and needy.
Kylux - Abandonment issues + Needy!Kylo