Sammi is like most other girls, she's insecure which leads to a lot of her manipulating and controlling tendencies. I doubt she has BPD and that she's as evil and shady as the editing tries to make her. Most of the other girls had bf's at one point and were being shady but the show obviously didn't focus on it.
Ronnie also suffers from insecurity, along with being immature and having a lack of self control. He clearly has a case of oneitis which is only encouraged by the fact that they live together. If they never lived in the same house I doubt they would have lasted this long.
Like most with self-esteem issues, they find it hard to obtain happiness and assurance by themselves and rely on another person to make it for them.