Stupid Boy Syndrome - sometimes referred to as SBS - is a common disease in high school boys. Boys who suffer from SBS are clueless on how to treat their girlfriends when they're in front of their friends or in general when they're not alone together. However while alone with a girl they feel the need to open their hearts and then they treat them like any normal, sweet, benevolent guy would. There are few boys who don't suffer from SBS, and those are the lucky few.
At school Mari confessed her love to Andrew and since he suffers from stupid boy syndrome he didn't know how to take it, but ignored her. But when they were alone in her car, he confessed his love to her.