Allergies and people who don't believe in them

What is it with people who immediately start doubting other people's allergies as soon as those allergies inconvenience them in any way? I mean, I can understand people saying - or thinking - "I would like to do X, and I don't give a flying fuck if somebody is allergic to some product of it", although I don't generally approve of this attitude, but what a lot of people actually say is "I would like to do X, and the person Y who claims to be allergic to some product of it is surely not really allergic and is probably just a hysterical attention-seeker". Do people really think that saying the latter makes them sound less like assholes than saying the former?

This particular rant is inspired by several people whose friend was having her hair colored with henna, which caused one of the people present there (not their friend) to open the salon's door in order to avoid an asthma attack (their argument was along the lines of "how can he be allergic to henna if he is not allergic to ammonia"), but I've seen this kind of attitude many times. I've had my allergies to tobacco smoke, marijuana smoke, and cats questioned many times by people who smoke tobacco, marijuana and own cats (luckily a rather small percentage of smokers and cat-owners), and my allergy to onions questioned by relatives and restaurant staff who tried to feed me onions. I've seen people say that nobody could possibly be allergic to their cat since people don't usually notice a concealed cat. I've seen an awful lot of people claim that nobody could possibly be allergic to peanuts enough to have a reaction without ingesting any, in the face of rather overwhelming evidence to the contrary. (No, I am not allergic to peanuts in case anyone is wondering, but many people are.)

Funnily enough, I have never seen a person doubt my or anyone else's allergy to birch pollen. Or any other pollen, for that matter.