My answer is something along these lines, does this sound alright?
I don’t have some child-hood story and I haven’t always dreamt to study astrophysics. I’ve enjoyed my academia alot and feel as though I have more to offer as an independent researcher, I have always wanted to do a PhD- I did my final project undergraduate in general relativity and I was grateful to finish on the studying the most beautiful theory I had seen, beforehand I knew very little about general relativity. The current cosmological constant model being based on general relativity, I realised perhaps I could study astrophysics or cosmology. I’ve since been very excited about this and there can be no field more exciting than addressing the fundamental questions that have fascinated mankind for years and researching phenomena occurring light years, astronomical observations continually discover new situations, as a result of revolutionary techniques. To increase my background knowledge and hope of pursuing a PhD in astrophysics, and after very much enjoying general relativity, I have chosen to do my masters level research project in modified theories of gravity and cosmology.