Chocolate contains caffeine, so it acts as a stimulant and increases the heart rate. A study from the National Center for Biotechnology Information also finds that chocolate has a protective effect on the cardiovascular system; however, this all depends on the type of chocolate consumed as well as the amount.
BBC News reports that Dr. David Lewis of Mind Lab, and formerly of the University of Sussex, found that letting chocolate melt in the mouth produces more intense and longer-lasting results than kissing. In Lewis' research experiment, the chocolate nearly doubled the heart rate and gave participants a longer-lasting "buzz" than volunteers who only engaged in kissing. Lewis' study suggests that allowing the chocolate to melt contributes to the long-lasting effects, but the caffeine already gives chocolate its ability to increase the heart rate. Psychologist Sue Wright also explains, "Chocolate contains phenylethylamine which can raise levels of endorphins, the pleasure-giving substances, in the brain." Lewis includes this information in his study to suggest that the combination of the endorphins and caffeine contribute to the physical effects chocolate has on the body. Lewis also suggests that these effects contribute to addiction to chocolate and why it produces a buzz in consumers.