I had the same questions, and personally felt that smoking was worse than not-so-serious si, since smoking harms not only the smoker, but also other people and pollutes the environment. I had this mild desire to be able to just go out and show my scars visibly without other people giving it a second look.

I kind of realized the difference when some of my friends saw my fresh cuts last month. One of them said sure, they may just be cuts, nothing serious, but what they were really concerned about was that I could do that to myself, and possibly worse things in the future. I think maybe what makes si different from smoking and other more accepted stuff is the intent. We actually want to hurt ourselves. I don't know.