There is more to love than just experiencing the rush of endorphins and phenethylamines flooding your brain. Faith/Trust, mutual respect and learning how to comprise to each other needs when necessary are just some of the key considerations in maintaining a long lasting relationship.
Anyway from a guy's perspective, this type of girls are just insecure about themselves. Threaten to end the relationship and chances are that will cry father, cry mother and promising to change for the better (for a short while at least). In extreme scenario, they might even take the kitchen knife and stab you in your heart repeatedly for 99 times.
Also, please kindly refer to post no. 08 in this thread.