Some hard-working psychologists at the University of California, Santa Barbara found that although tickling generally elicits laughter, it’s not always funny or pleasant. Here’s an excerpt from a New York Times article about that study:

The basis for the newly published study is what is known as the warm-up effect, the scientific underpinnings for the phenomenon of the warm-up comedian. If a person finds something funny, researchers have previously shown, the next thing encountered will seem that much funnier because of an already giddy state.
So one group of students was tickled for 10 seconds, or until the tickling became intolerable, and then shown videotapes of stand-up comedy routines and clips from ''Saturday Night Live.'' A second group watched the comedy video first and then was tickled. A control group watched a patently unfunny nature video, then was tickled.
Researchers postulated that if humor and tickling are related, and the warm-up effect applies to both, then subjects should laugh more when tickling follows humor or humor follows tickling. But that was not the case. Tickling, the study suggests, does not create a pleasurable feeling -- just the outward appearance of one.