This is exactly what happens when you tell your friend a secret and especially ask him to not share it with anybody.
Because the moment we ask them to shut their mouths, that’s the moment when they open it wide like a Hippopotamus :p
It’s a basic human tendency to actually do those things first which we are not allowed to because we feel it’s very adventures to do so and we can’t resist the tempting.
Specially when there is a chance to bully our best friend in public! And we all have one such friend who just cannot keep a secret.