Why dolphins likes to jump out of the water?

Dolphin jumps out of water may have several reason:

1: breathing as they are not fish and they muts breath in air
2: to relieve the itches on their body caused by the parasites
3: a kind of playing behaviour....

It's difficult to tell which scientist find that as it was mentioned by a lot of scientist......
and actually a lot of dolphin has this behaviour such as Chinese White Dolphin (Sousa chinensis)

The one above has good answers. I only want to add that jumping out of water is a way to peek far away places, like submarines need to use a periscope to see what's going on above water surface, especially when the water is muddy.

One more possibility is that they are communicating. From the TV series Blue Planet, a few dolphins found a big school of tuna fish and cannot handle it. They then jump out into the air, signalling friends to come over. Then other dolphins joined to round up the fish and everyone have a nice meal.