There are many reasons why tickling is bad, and considered a form of torture in some countries. 

Reason the First:  Tickling causes a lack of oxygen to the brain.  This is evidenced by the shortness of breath observed in victims: the flailing to get away coupled with the screaming leaves the victim tired and defeated.  The physical consequences of being tickled will be explored more thoroughly in forthcoming Reasons.

Reason the Second:  Tickling is rude and should not be used as a way of motivating, cheering up, or as a form of greeting.  It is considered impolite to tickle your guest and violates the idea of "tutus hospes" ("safe guest" from the Latin).  It is also unacceptable as a form of saying goodbye.  The one time that tickling is acceptable in a social context is never.

Reason the Third:  As stated in Reason the First, there are several physical consequences caused by tickling including measles, whooping cough, and chicken pox (as evidenced by the large number of babies tickled by cruel parents).  Short term effects include anxiety, loss of motivation, and hangnails.  Long term effects include mania, lobotomy, and the desire to own many cats.*

Reason the Fourth:  There has been a high correlation between People Who Tickle (heretofore to be known as PWT) and members of the Whig Party.  Several prominent Whig members have been members of People Who Tickle and We Just Don't Care (PWT&WJDC) and as most civilized people are aware, the Whigs were responsible for the Teapot Dome Scandal, the Boston Tea Party, as well as the force behind a number of stolen tarts and crepes throughout the decade.

Reason the Fifth:  Most important to remember, tickling is not a joke.  This stereotype of the "friendly tickler" is a myth that should not be perpetuated.  Most people who tickle are unrelentless and evil and not to be trusted.  Tickle-eee's should rise up and fight the tyranny of PWT.  Only through education and outreach will we ever be able to overturn this often overlooked aberration of human behavior.

Rise up and fight against Tickling.  Remember the best offense is a high pitched scream at a loud volume.  

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*In 1986, there was a recorded incidence of tickling inducing the desire to own many hamsters instead.  This case study is more formally covered in the scientific journal "Inhuman Gestures" and followed the case of Darlene X, who after years of tickling was impelled to adopt as many wild hamsters as she could.