Relationships in general make people a bit nervous. It's about trust. Do I trust you enough to go there?
If you can't go 5 minutes without checking Facebook on your phone, we're probably not a good match. However, if you enjoy having thought-provoking conversation and aren't afraid of the occasional spirited debate, give me a shout!
People could rationally decide that prolonged relationships take up too much time and effort and that they'd much rather do other kinds of things. But most people are afraid of rejection.