The reason why she says it is because when he finds -or tries to- find someone new, he wouldn't know what he's looking for. If he's trying to replace her or find someone else to fill in and gets lost and confuse on the way. The next girl may not be what he wants and he'll struggle and hurt that person... Eventually he'll become so exhausted that he'll go back to her, his past. Fact of the matter is that he's been with her so long that he can't forget how perfect she was for him. He'll miss the person that he was when he was with her and all the great times they had together during that time.
I know all this cos it's happened to me, couple of times. And now I'm with someone whose had a past... I won't let my guard down. So hopefully that means I won't get as hurt as before... I just hope for the best cos I want to give my all in this relationship. Fuck me man, this always happens to me.