Butterflies are from body's fight or flight response. You often get them in a negative way if you have "performance anxiety," in situations like meeting new people or public speaking. It's a strong physiological response involving adrenaline and your adrenal glands (located above your kidneys), and it usually means you're way into someone and are concerned about impressing them, i.e., they have signalled strong social value to you.
The emotions and the body's (and brain's) chemistry are very interrelated, and all three are related to your health. When you are first in love with a new person and begin to experience their touch, etc. you get overloaded with endorphines like Dopamine and Seretonin, and these things do tend to cement the bond with that new person, creating quasi-drug addict behavior in the two of you. There are lots of good articles on these things if you just google:
endorphines, dopamine, drenalin, fight or flight, relationships, brain chemistry, frontal cortex, etc.