What does it mean when people want security in relationships?

What does it mean when people want security in relationships especially since we all individuals and can't snap our fingers and automatically protect those we love cuz we not with them all the time and can't control all situations. So what does it mean when a women is dating different guys as a high school or college student and always showing affection and getting attached and wants to feel secure? Doesn't she get that from her relatives and her own family? I've just wondered about that a little bit. Sometimes I think guys just want the relationship for the romantic affection while the girls want it for the co-dependency and acceptance, but I could be wrong. I just seen that stuff since middle school with couples getting together. Hmmmm.