[jeffaziz]

jeffaziz  asked the Naked Scientists:
   
Chris,

My daughter Samantha would like me to ask you, "Why can't you tickle
yourself?"

Thanks,

Jeff Aziz

What do you think?

[RD]

Because you know it's coming.

The tickle respose is a protective reflex to draw your attention to, and cause you to retreat from,
 potentially harmful things you were not aware of moving on your skin, (like spiders, insects, rodents, snakes).

The D-I-Y tickle fails because you know a stimulus is about to occur.
 Not expecting or being able to anticipate exactly where the touch stimulus is going to occur is necessary for the tickle response.

[TonyCap]

I did a test with my son.  I let him know the tickle was coming and yet it still caused him to laugh.  How do you explain that?

[RD]

I did a test with my son.  I let him know the tickle was coming and yet it still caused him to laugh.  How do you explain that?

It is reflexive, telling someone "I am about to tickle you" will not switch off that reflex.

I bit like telling someone you are about to shine a light in their face but that they should not constrict their pupils.
 It's not under conscious control.

When you scratch your neck your brain initiates this stimulus, so expects it, so it doesn’t trigger the tickle reflex. If the sensation on your neck was unexpected by the (subconscious) brain it could be something nasty like a rat about to sink its teeth into your jugular, so the protective reflex is triggered to escape the possibly harmful source of this unexpected stimulus.