i think its because they have no respect for each other. its become a very toxic relationship which she tries to hide by saying its banter. at the start of their relationship sean seemed very loving, all the old pictures and videos he is touching her and smiling at her etc now he just makes moaning digs constantly. shes made it his whole personality that hes blunt, honest and a moaner so he clearly has ran with it. to other people it just comes across really rude and disrespectful. i get the impression she, once again is copying her mum. shes copying her mums banter that she has with her partner. the difference is being that theirs is genuine.
to be honest though her and Sean suit each other. hes literally lowered himself to her chav family standards.