I think it's just because the "loop" in which Joe kills himself is that final loop, and something like killing someone is irreversible. But who knows, another Looper that knows Joe could come back and kill Joe before he kills that boy, and so in that loop the boy will live.
Basically there are many loops with many different versions of our world, depending on certain events. Since time travel isn't real, the director and writers can do whatever they want to do like the writing on your arm, will suddenly be engraved on the future version of yourself. In actuality, if time travel were real, the carving on the arm would have always been there, it wouldn't just magically appear as you're writing it. But this is up for debate as well. It just makes the most sense to me.