As simple as it sounds. Sometimes our kinks are mysterious, but sometimes you can look back and see a clear origin for them, whether that be a traumatic response or a childhood hobby or something.
For me? While figuring out my bisexuality, i also played a lot of fighting games.
Seeing two female characters throw each other in Soul Calibur or Dead or Alive was one of the earliest examples of female physical contact that i got that my parents didn't immediately try to ban so like... it stuck with me and now as an adult i get turned on by certain kinds of fights or martial arts gear.
Both my kinks of female orgasms and breeding/creampies, came from my first sexual relationship. We were young and she was anxious about pregnancy so I never came, even with a condom. I developed good control and learned a great deal about giving pleasure. Kink #1 unlocked was the of her cuming because of me. The creampie kink came about out of desire for it.
The girlfriend after her was the first person to let me cum. Holy shit that awoken something deep and primitive inside me. If she's cumming because I'm coming inside her, I'm in heaven.