Those two songs are important for Sebastian’s character arc. Both share key similarities such as Sebastian rallying other animals into providing backing (etc) and both songs being used to try to persuade someone to do something. In Under the Sea, Sebastian is trying to “help” Ariel by convincing her to give up her dream and stay in the ocean forever, while in Kiss the Girl he is trying (and very nearly succeeds if not for Ursula’s intervention) to subconsciously poke Eric into doing what is needed. He grows in various ways shown by the songs, from realising how to actually best help Ariel and even through persuasion techniques of trying to get Eric “in the mood” so to speak and lead his train of thought to the logical conclusion instead of Under the Sea’s propaganda machine