Bottlenose dolphins are the most common breed of dolphins used in aquarium shows due to its intelligence, charismatic 'smile' (due to its curved mouth) and friendly character. It ability to perform many tricks has gained the hearts of many people especially children.
Furthermore, there has been many incidences whereby humans were rescued by bottlenose dolphines (the most recent one was when a surfer was rescued by a pod of bottlenose dolphins from a shark), and thus they gain the title of 'men's best friend in the sea'.
Interestingly, the bottlenose dolphin now rescues whales too!!
On March 13th 2008, a bottlenose dolphin, named moko, rescued 2 pygmy sperm whales stranded in the New Zealand beach. Conservation workers tried to save thw whales but to no avail. But moko managed to communicate with the distressed whales, leading them out into the open sea. As mentioned, moko is a famous dolphin among the locals as it interacts with those at the beach. so it seems like moko is not only a playmate for the kids, but also their guardian angel.
3 interesting points I want to highlight:
1) bottlenose dolphins do not mix together with the pygmy sperm whale. They are usually seen interacting and travelling together with the Pilot Whale.
2) It saves 2 whales! It is impressive to know that they often save distress humans, but now it spreads its helpfulness and righteousness to the whales!
3) The 2 pygmy sperm whales were willing to allow moko to lead them out. It comes to question that dolphins and whales communicate using their 'whale language' ? (eg. the whistling sound)
The Bottlenose dolphins are often seen interacting in mixed-species groups, most commonly with the pilot whales, pantropical spotted dolphins and spinner dolphins. Occassionally, when the 2 pods of bottlenose dolphins and pilot whales interact, they became more active, playing and surfing on ocean swells. The Pilot whale is divided into the short finned and long finned species, displaying in similar interlligence as the bottlenose dolphin.
Several years ago, a group of mix bottlenose dolphins and pilot whales were spotted being together with a group of Sperm Whales. The sperm whales were observed to be behaving unusually. The group of bottlenose dolphins and pilot whales accompanied them thru-out. Although what was happening was unknown, it seems that the dolphins were trying to protect and accompany the sperm whales... (more)
As quoted from a website "Bottlenose dolphins may aid ill or injured pod mates. They may stand by and vocalize, or they may physically support the animal at the surface so it can breathe". Therefore, this may suggests why they will help those in need, irregardless whether they are humans or other cetaceans.
So it seems like the bottlenose dolphins not only are friendly cetaceans, within them, they have a sense of helping those in need! 
References
"Bottlenose Dolphin" Last modified: Thursday September 24th, 1999 http://ourworld.compuserve.com/homepages/jaap/tursiops.htm
"Dolphin's Encyclopedia: know the short finned Pilot whale" http://www.t-d-e.org/knowshort_finnedpilot.php
"American Cetacean Society Fact Sheet: Pilot whale" http://www.acsonline.org/factpack/PilotWhale.htm
SeaWorld/Busch Gardens Animal Information Database http://www.seaworld.org/infobooks/Bottlenose/behavdol.html
"Dead Baby Pilot Whale" by Katharina Heyer on September 5, 2003 http://www.firmm.org/en/aktuelles/news/030905-grindwal.html
Image source
http://www.hitech-dolphin.com/image-files/information-on-bottlenose-480.jpg
Video Source
http://www.youtube.com/watch?v=Fp_motddvnQ
http://news.nationalgeographic.com/news/2008/03/080313-dolphin-video-ap.html