Happy Birthday to You

Happy birthday to You...

Such a simple and short sentence and could only be used for each person once a year...
what does mean?

Does it really mean to the happy birthday?
Does it have some hidden meaning to transfer to the one which will receive this?
Did someone have consider about the spirit of the sender?
Did you catch some unusual feeling about that?
.....
...

Sometimes, it could mean great friendship between indeed friends.
Sometimes, it could mean the deep sensibility between the relatives.
and sometimes, it could mean the LOVE...

Happy Birthday to You...