In most media, especially most fiction, dolphins are portrayed as playful creatures, who will easily befriend humans and like to play around in the water with them. In real life, the extent of this varies. Dolphins can be trained to be like this, but one shouldn't expect this from wild dolphins, who are often hard to predict. Hence, experts recommend keeping your distance from them. This trope wouldn't have gotten its popularity if not for applying rather well to some dolphins, but the dolphins it's based on are the ones people see in aquariums and zoos; whom the label "trained" tends to apply to. They aren't going to be a representative sample of dolphins as a whole.