What's happier than happy, gladder than glad, more peaceful than peaceful? We're not really sure, but the speaker's sure experiencing it.
Quote #5
Because the birthday of my life Is come, my love is come to me. (15-16)
These concluding lines let us know that the speaker's happiness is rooted in a profound appreciation for her life. She's not simply in a good mood; her entire existence is validated. That's some intense happiness.