Tickling is kind of odd, it is extremely hard to control for most people, and yet, you can’t really tickle yourself.
The reaction you encounter when tickled is actually a panic and anxiety reaction, it is a form of defense against possible crawling insect that may be poisonous for example. So the body needs to react quickly to avoid any risk.
So why can’t I really tickle myself? Well that is because you won’t feel in danger knowing you are the source of the tickling and that you control it.
Interesting enough, the reaction is not the same if you are tickled by someone you actually are OK with tickling you, or not, probably based on the level of confidence you have toward this person, thus adjusting the level of danger your body feel and the reaction to the tickling.