Insecurity sucks. No one likes an insecure person, and no one wants to be an insecure person. But it happens. Sometimes it's not justified and sometimes it is. She might be behaving irrationally, but who knows? Maybe her boyfriend doesn't act like he's attracted to her, maybe they don't have enough sex, maybe he talks about other women too much.
I know nothing about their relationship, but it's not always something inherent in the girl that causes the insecurity, so I wouldn't judge her too harshly. Usually people grow out of it, (and then it all comes back when they start aging and their husbands start going to strip clubs and getting lap dances from girls 15 years younger - sorry, just projecting here).