I don't think that's true at all.
when you're a teenager and you start to date for the first time, it feels like it's going to last forever. your inexperienced mind thinks that you'll stay with that person forever, marry, procreate, and then die...
but it's not that people THINK that teenage love it's just a experience, it's what actually happens a lot of times.
teenagers are in their most volatile and vulnerable stages of their lives, its inevitable that they're going to change their minds about their "soulmates"