Why do dolphins jump up out of the water?

Dolphins jump for MANY reasons, spinner dolphins for instince are very energetic and spin when they jump, they jump for fun, other dolphins may use this as a signal to other dolphins that they need help with catching sardines etc. Some dolphins jump because it is easy for them to look ahead when they are higher than the sea, they may use it for navigational purposes, sometimes just jump to express hapiness and their excitement, or they may jump because they are full of energy and want to play. Dolphins sometimes use it to communicate things to other dolphins, other times for hunting and some may jump for the sake of jumping. =)
To breath or for people to see them.