The human brain is hugely suggestable and psychologist probably could rule the world.
The second you are told you can't have/do something you REALLY want it out of all proportion to a rational need.
For example, I just read PP "that takeaway coffees are hugely overrated" which normally I'd agree with but now am craving a large sugary Starbucks coffee but only because I can't get one. In normal times I'd just be meh I agree totally.
Don't get me started on Long Island Ice Tea.