When you sing the happy birthday song, do you say
happy birthday dear (person's name)
happy birthday (person's name)
You have to use dear to fit the traditional tune of the song, otherwise it's missing a note. Additionally, the most commonly used version you hear in America has dear in it, even if it makes the song awkward.
Happy birthday to you, happy birthday to you, happy birthday dear (name), happy birthday to you.
You may or may not use dear, as necessary to fit (or as nearly as possible fit) the metre. If a person has one or two syllables in their name, you need the extra syllable to fit. If a person has four or more, it would make it more difficult to fit the metre. However, some are so much in the habit of using dear that they use it even for longer names.