Theories of Emotion: Schachter's Two Factor Theory Example
I think it's because he interprets his emotion as anticipation and excitement because of the context - because he's about to go skydiving which is fun and exciting.  If he was at home watching a boring TV show and had the same physiological experience, he might think he was having a symptom of some kind of heart disease.  If he'd just tried a new drug and had the experience, he'd think it a side effect of the drug.  If he'd just met a new person and had the racing heart, he might think he was in love.  The physiological response (racing heart) is necessary, but it needs to be defined cognitively according to the context (skydiving), that's the two-factor theory.