One tactic you might try is not stopping at the first level of thinking; ie just answering "Why do I want to go?"
Instead, once you've detailed the reason(s), go on to answer a deeper question that is more personal. For example if it's the desire to serve as your motivation, then...
"What does serving my country mean to me personally?" or "Why is serving so important to me?"
So just don't stop at the easy answer. Look deeper into yourself and share with the academy how/why you feel that way and what it means to you. That's where variety comes in. While many people applying have a desire to serve, the reasons behind that desire, how they view that service, what it means to them, is where it becomes invidualized.
Hope this helps and good luck!
NOTE: Some of the more veteran posters here may have other advice you should listen to, but the above is what my class of 2017 cadet did.