Surprise is a big element. Like when two people meet in an unexpected place and just click. For example, at a grocery store where you don't expect to meet someone and start a great conversation that leads to more.
The spontaneity of the encounter matters a great deal. A hookup that happens without any pre - planning, like two strangers sitting next to each other on a long bus ride and getting to know each other throughout the journey, often makes for a great story. It's that natural flow of getting to know someone new in an unplanned situation that can be really special.