We all sometimes want to go back in time change certain things that have happened in the past but it simply is not a good idea. Apart from what someone said about learning experiences which I guess you make a fairly good point. But the real reason is that time is not set in stone. This is because the universe is constantly duplicating itself every time an atom has a choice of going one way or the other. So by now there would be millions most likely alot more but anyway alot of different universes. This proves that things are not set in stone. And another reason why we should not travel back in time is yet another paradox to be solved. Say ,for example, that you go back in time and stop JFK from being assasinated. Now I like the sixties prez as much as the next guy but what if something about his assasination caused your parents to meet. Now without the assasination your not alive. Or at the very least living an extremely different life because your parents may have somehow still met and your life went a different direction. This is a simple reason why we should not travel back in time. Now forward is a different story. I believe that even if you drop a nuculear bomb in the future there will still be a way to change it by doing something else when you return to the present.