Imagine being in a relationship with Luke, but sadly you guys decided to break up; which literally tears Luke apart, as you meant just as much as 5SOS did to him.
Even to this day, Luke is still torn up about it; blaming himself saying that he should've been a better boyfriend, should've spoiled you, made you feel like a princess etc.
The boys try to reassure Luke by saying that it wasn't his fault that you guys broke up.
But that doesn't stop the voice at the back of Luke's mind from telling him that it was all his fault...