Mouth ?

I never had a cold sore until one day, thinking I was invincible, I shared food with someone who did. Now, every so often, they appear. About a week ago, I got the first one I'd had in about a year, coincidentally, about the same time that my little sister had one. How come it took so long to get another one if I'm doomed to always have them? And is it possible I caught it from sharing something with her? Is it also possible that I'll only get them from other people, since I wasn't born just... having them? And, one more question, since I didn't get one until it was given to me, will my problem be hereditary?