Friedrich Nietzsche was a proponent of the concept of eternal recurrence. Essentially, everything that has happened and will ever happen will repeat exactly as it has and will all over again for all eternity. Similarly, physics describes how reality functions on different dimensional planes. We exist in four dimensions, but if, for instance, we are observed from the perspective of a fifth dimension, time would appear flat and circular. Every moment and place within our four dimensions would exist as a superposition of cause and effect, all of history playing out in one instant forever. So, there could be some scientific basis for the concept of eternal recurrence. However, that isn't the point. Nietzsche was a proponent of the idea, but only because of the opportunity it presented for the ultimate affirmation of life. He asked the question: "What, if some day or night a demon were to steal after you into your loneliest loneliness and say to you: 'This life as you now live it and have lived it, you will have to live once more and innumerable times more' ... Would you not throw yourself down and gnash your teeth and curse the demon who spoke thus? Or have you once experienced a tremendous moment when you would have answered him: 'You are a god and never have I heard anything more divine.'"
Why am I alive? As an existential nihilist, I do not believe that life has any inherent meaning, but we, as conscious humans, have to give it meaning in order to affirm our existence and struggle. I am alive because the meaning I have given to this life is the search for that one moment that would make me answer the above question with nothing but joy, the singular moment that would make every single negative worth repeating just so I can experience it again and again forever. Maybe I will never find that moment, but I exist to try.