I was eating popcorn while reading the answers.
Getting serious:
NPTK Is a love song? Yes it is and there is no point in beating around the bush.
The real question that needs to be asked in my opinion:
NPTK Is just a love song?
Here the matter is more complex because as you can easily imagine, no it is not and it will not be for at least the next 3000 years.
NPTK is a song about love yes but what kind of love: romantic love lost, love of a family, love of friends;
all these differences of expression are however a form of love.
NPTK is about hope: the hope of being reunited with loved ones.
NPTK is about awareness: whatever you say, Aerith knows what it means to be the last Cetra and what this could mean for her life.
NPTK deals with the theme of remembrance: we will always carry loved ones in our hearts who for a reason are no longer with us.
NPTK is about farewell: it's the theme of Rebirth. Goodbyes are part of the cycle of life but, reconnecting to the theme of hope, these goodbyes will never be goodbyes if there is hope, reconnecting to the theme of memory, it will never really be a goodbye if we carry our dear within us.
Philosophical moment, switch off.