Here's the psychological viewpoint. When anybody is first attracted to somebody, it's a physical attraction. The two people don't usually know each other emotionally, so they're working off of physical attraction to get the relationship off the ground. However, as their relationship grows, and they get to know each other, the two become emotionally attracted, and that's where relationships become serious. My answer is that attraction starts out physical, and grows to a point where physical attraction and emotional attraction balance each other out and work as equals.