At some point in your life, you've probably been in love. If you haven't yet, you will be. Your heart starts racing, you palms might start sweating... but what is it exactly that makes your body respond this way? It all has to do with your body's chemical reactions.
When you first happen upon that special someone, you start to feel romantic, passionate, giddy and euphoric. These feelings aren't purely psychological. If we were to take a blood sample, we'd discover high levels of certain chemicals in your blood. The first of these is dopamine, which is also activated by drugs like cocaine and nicotine. The second is adrenaline, the hormone that your body uses to respond to emergencies. When in love, it's not uncommon to be a little hyperactive.
No need to worry. As with any stimulant, eventually you'll develop a tolerance for the object of your affections. When that happens, all this ecstasy will be replaced by the mellower feelings associated with long term relationships, feelings like peacefulness and security. If we were to take a blood sample during those especially intimate moments, we'd discover high levels of oxytocin, the same hormone that promotes bonding between mother and child during nursing. From the perspective of evolution, this bonding between adults has an important function: it keeps couples together until their children are able to take care of themselves.