Dolphins are known for being highly intelligent and social animals, so they can also symbolise wisdom or smooth communication. Just like the ultrasonic sounds dolphins make when they're happy, it's also used to show extreme excitement.

K-pop fans sometimes use this emoji to cheer for idols who hit high notes well or have a bright personality. There was also a fun trend of calling shiny short shorts 'dolphin pants', like the smooth skin of a dolphin.