You know that the bliss they feel when they're 'in love' won't last. You understand that they think it'll be forever, but you know otherwise. It can be a pretty scary thought knowing that something so blissful for your child has the potential to work out so badly.
You don't want to potentially spoil the relationship you have with your teen by handling their heartbreak badly. However, you also don't want to make them bitter about love or too caught up in the 'forever' moment. The balance has to be just right.
I came across a fine article about teenagers and heartbreak. It gives really good advice on what to do when your child has her first break-up and the aftermaths of it. Advice like not 'bashing' the ex or to be prepared for a relapse (and lots more) are very well expressed. You can go over and read it here.