John Lilly, who spent years working with dolphins, believed that they were smart enough to be cute and friendly to humans because they understood how dangerous we were.
Most of the time the people think about how cute is a dolphin is misunderstanding the animal. If a Lion shows you the teeth, you don't think is a cute kitty, is angry!, but, hey, this dolphin that show us the teeth "is smiling"!. This "smile" is in fact an agressive display in the wild.
Being faster, well armed, and better fitted for water, a dolphin can avoid dangerous humans in the water any time he wants. In fact a lot of alone exemplars search for company, actively, even if is from humans, and are curious. Some dolphins simply like the human company, or the benefits derived of this, other not... at all.