It really is

YOLO is "you only live once" and means to live life to its fullest each day.
Carpe diem is Latin for "sieze the day" and means live life to its fullest each day.
Same meanings but, if you ask me, carpe diem sounds more intelligent. Not just because it's Latin, either. I would rather see people wearing "sieze the day" shirts than shirts that say "YOLO".
Then again, I guess they use "you only live once" because if they abbreviated "sieze the day" you'd get STD.
But then again, why the hell must everything be abbreviated?
Also, you live only once is the grammatically correct way to say it

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