If it goes too far, tickling can become life threatening; not because of the tickling but instead the health issues of the ticklee. For example, I have high blood pressure, an enlarged heart, and kidney issues (lost a kidney to cancer in 2008). To allow myself to be tickled beyond my limits, it would be just as potentially harmful as anything very stressful or physically overactive.
I also was born with asthma, and know from experience from childhood to today that anything which causes me to laugh hysterically for minutes on end or repeatedly will start me to wheezing....and not stopped, can cause shortness of breath, etc (and nothing is more frightening than panic from being unable to breathe).
I love tickling and being tickled just as much today as when I was 19.....but realize sometimes you must listen to your body; there's no way I could take the same amount of tickling now as three decades ago without ramications to my health (and why I use common sense when tickling others; the last thing I'd ever want to do is cause someone to faint, lose their breath, or suffer a heart attack).