Why isn't sudden water temperatures good for pet fish?

[luvmigoldfish]

I was changing my goldfish to another aquarium and i wasn't thinking ,so i found out that they like cold water .So i used cold water for the new tank,and they were coming out of a warm ,a very warm tank, and in less then ten seconds i dropped them in the cold tank. I felt so bad i wondered was this okay? Are there health problems dealing with this. I know they are just goldfish,but they are my baby's and i love them. So, somebody please help,but there okay i just want to know more information about how sudden water temputers are good for fish.