This, to me, is one of the most important things to say when someone is going to college, going abroad, getting a new job, or doing anything new that everyone assumes will be a great experience. Looking back now, I would even add that I'll support the person if they change their mind and decide to return home. It's a hard topic to bring up because you don't want to talk about things going wrong when someone is excited, but you can think of it as preparation. No matter how excited someone is about going on a hike, they should still pack a first aid kit in case someone gets hurt. You're not taking away from your friend's excitement - you're being part of their back-up plan.