Teens still have brain receptors forming until nearly college age. Honest to goodness their brains are just not right yet. It may cause teens to take things too seriously or too lightly. That is why so many teens do stupid things their brain knows they should not do. It may cause them to take first love as the end all be all without considering very few people end up married to the first person they ever dated.
It's all about perspective. Kids grow up thinking that teenage love is incredibly important, and there is a bit of a dreamy quality to it. Some teens get fixated on the "need" to be in a relationship in order to be socially accepted.
TV shows don't help either, despite some shows (like Degrassi) showing how there are pitfalls and obstacles even to teenage relationships.
It's all about exploring yourself and making decisions, and learning from them :)
Because they don't know any better and when older people tell them, they don't want to listen because teens think that they know it all or maybe they just act like they do. But with time they learn. Older people have done the same things and have gone thru the same issues.