I know. I agree with you - it's really exciting and cool when someone you know appears in your dream. You really want to go and tell them all about it, don't you? But I have to tell you... It is in your best interest just to keep it to yourself.
Maybe you should treat it this way: Imagine that the person you dreamed about has an extra personality in their head that they're unaware of. It takes over for them at night and comes and hangs out with you. You do all sorts of cool things together, but for some reason, if their 'real' personality ever finds out about it, the universe will be destroyed. See - now you can't tell them. It's that easy.
Just cherish the time you were able to have together and leave it at that. I'm serious. If you tell them about it, they will probably try to act like everything's cool but secretly they will hate you and think that you're a nut or obsessed with them or 'clingy' or some crap like that.
You don't need to thank me.