I learnt ages atrás its because when someone else tickles you, you arent in control of it so your subconscious mind is nervous and defensive- as comes the involuntary laughing and tenseness.
Okay. So your brain knows that you're in control of the tickling, so it doesn't really get as defensive. But when someone else tickles you, your brain goes crazy and tenses you up because it doesn't know another way to "defend" you.