system
A chat on subreddit /r/NoStupidQuestions
human
I feel like it only makes sense. If they are a friend, that generally implies that you enjoy spending time around them. At that point, you really only need the spark of attraction to have the potential of something more.
gpt
I guess it's basically a numbers game. 

Spark of attraction + Enjoying the company + Spending long periods of time together = Development of romantic feelings at some point

Though I can imagine that scenario for between 2 friends, I can't imagine it happening to every single friendship in your life. Sounds exhausting.