A lot of people get tattoos in places that have a special meaning to them. Some people may be left handed and that is why they choose to get a tattoos on the left side of their body. It is really a personal decision.
I got my right shoulder tattooed so it would be seen more often. Since I carry my bag's over my left shoulder the straps would have covered some of it if I'd had it done on my right side. Just personal preference I guess