It's a bit of an old school joke about them being caught through kissing. My ex constantly got them, but I have never had one in my life (and we did kiss a few times!) Unfortunately some people are just more susceptable to them than others. It's a bit like acne and spots, some people have perfect skin even though they eat crap, and others have the healthiest diet but still get acne.
I guess its not a good idea to kiss someone if you/they have an open sore as the virus will be active.
A doctor I work for has told me that everyone has the virus in them, for most it remains dormant but for others it flares up. Being run-down is definitely going to make you more open to getting them though.