If someone tickles your armpit, you won't laugh as long as you stay relaxed. Of course, it is not easy to stay relaxed at this time. Most of us feel nervous when others scratch ourselves. Because you will feel uncomfortable with this kind of physical contact, you can't control it, and you are worried about itching or pain. Some people are not afraid of this because they are not nervous for some reason.
Catch yourself, everything is under control. You don't have to be nervous, so you won't have a natural reaction. You can put your hand close to your eyes, but other people's hands can't. You must close your eyes for the same reason. The next time someone tickles your armpit, you just need to keep breathing well and relax, so you won't giggle.
Laughter is the result of being in a state of slight fright. This feature does not seem to conform to the evolutionary theory of "survival of the fittest", because panic will make you vulnerable. However, there are many examples that show that not everything in nature is logical.