If your first sexual experience is with a hooker, you just might "fall in love" with her. The physiological power of sex is so convincing, you might overlook the fact that you keep paying her cash afterwards, and confuse this physiological pleasure with love.
It's the same reason you probably said "I love you" to the first person you slept with. Looking back, did you? No. A more accurate admission would have been to say, "I'm addicted to you."
Your friend finally breaks up with a girl that was so completely wrong for him and says to you, "What was I thinking???"
Sex is addictive. But, if your goal is long-term relationship, you really need to wait until marriage. Because over the long term, it's much more important to be addicted to the person.