People or more specifically we can say that couples use the term or the phrase happy birthday sweetheart while wishing their better half or their partner spouses for their birthdays and making the same more special and also memorable for the person on the other hand. 

Therefore, it can be said that the term or the phrase happy birthday sweetheart is being used by people all across the globe by love birds and partners in order to wish each other the happiest birthday. You can also use the same to your partner also.