Whats wrong with taking shirt off?

These days whenever a guy takes his shirt off people are like "put yer shirt on!" And so on. Anyway I'm 15 and been building muscle for...4 years now and I'm pretty ripped and me and my friends took our shirts off and I was even at Southport ( seaside ) on a hot sunny day and the girls there were like "oh stop showing off Sergei!" I wasn't even flexing or anything. So what's so wrong with it? Do girls not like it do they think I'm showing of? I mean even people in cars were braking there necks to look and people on the side walks would frown and give dirty looks especially the skinny guys! But when you go to America its usually acceptable to do it so if your in the U.K what's wrong with it I don't get it? Jealousy? You should take it off specially near a forest the air is pure there and feels refreshing on a hot day.