I got my first outbreak of cold sores when I was 15. I hadn't ever been on a date (catholic school upbringing), never shared any coke bottles or drinks with friends or families (paranoid family and cooties lectures from my mom), and hadn't shared a first kiss at that point with anyone. It was freaky- and as far as the doctors could tell, they just said sometimes they can come from stress of any kind. At that point, it was summer, so they just attributed the outbreak to the sun for me. I only really get them in the summer when they do happen.
Probably I did get them from some relative kissing me when I was a little kid or something, but they don't necessarily come out just because of a cold. Sheesh, they can come from any kind of stress on the body.