With different introductions and when Sims use it themselves, it would be nice to know why they use specific ones on certain people. One person said here that it's random and in all respect it shouldn't be. If a Sim uses a flirty or enchanted greeting on another, it should be because they fancy that person, the same way a real person would chat up someone they meet in a bar.
For example, a few weeks ago I was playing a household and when my teen Sim returned home from school, a message box popped up asking whether Samantha should invite over someone she met at school. Courtney showed up at the door, walked in and after talking to Samantha a little bit, she enchanted her. Clearly Courtney fancied Samantha, so that's why she used that particular introduction.