Probably because diff folks have different tolerances and manifestations of thc overload.  I am a woman in my mid 60s who has used marijuana for the past 30 years. For the last ten I have been ingesting the drug in the form of cookies or brownies, finding that this invokes a much stronger high and as a result, more for my marijuana dollar.  Insomnia being my main reason for using the drug, I found recently while I was away on vacation that the brownie did not help me sleep at all when I took it at my usual hour.  Thinking that I was just tense from travel, I took another one. I had the usual pleasant high feeling but an accompanying tense hyper feeling would simply not go away.  I lay in bed and shook, cried, and hyperventilated until I decided to cease the drug for the rest of the trip. It didn't work. I couldn't stop panting and my bp became sky high. (I can tell when my bp is up just by feel). i had no appetite, no strength, and no amount of meditation, yoga, tai chi or soothing music or hot baths had any effect at all. Also, I was suffering from depression which I rarely experience.  I was forced to go to a local walk in clinic where the doc took one look at me, and prescribed Valium, which as we know is almost impossible to get these days.  I took 2 of them (a ttl of 10 mgs) and finally began to calm down.  My plane was scheduled to leave at two that day, and if I had not seen the doc, I am sure I would have not made it.  I am now in the process of abstaining for weed until I am clear of thc, at which point I will attempt to take a few tokes and see what happens. I will be very disappointed if I cannot ever use the drug again, but I may just have overloaded myself to the point ft no return. Will keep you posted.