Why do some boys like to smell girls' armpits?

It's what you call a fetish. It starts from seeing something or accidently running upon armpits being on display. Maybe even from girls who always flip their hair and show their armpits available all the time. Or models who put their arms behind their head for a more lustful type of feeling to draw the audience in more. More than anything it's from giving the attention to it which magnifies it in the head.