I don't make them up, I just write them down.
That's what I always say about my stories. The words are mine, the characters are ... sort of mine, the plot is not. (Hehe, that rhymes.) I may choose my own settings, but with some restriction.
And I don't believe I'm the only writer to feel that way. Stories are curious things. They go where they will, and how they will. Often, they may look as if they're going to some destination, only to take a sudden turn in a new direction just over the horizon.
It may meet an impassable obstacle and be forced to go around. The road you were following may become unsuitable: too narrow for your vehicle, too steep for your engine, too busy and noisy to be pleasant, too rough for your feet or too soft for your tires. You may meet a new character who takes you in a new direction. It may just be that the story, for some reason you may never fathom, does not want to go that way.
The story may take you somewhere entirely new, or to somewhere that you had heard of but not thought you'd like to visit. You might stop somewhere you had thought to pass through, or carry on beyond the place you had expected to end your journey. You might just take a detour, or leave your route entirely. It could be that you are taken away from where you thought to go, a new direction by an unseen road, and then equally unexpectedly return to your original way, or reach your original ending by another path.
I thought that my vampire story would reach a certain point at the end of the first book, and would go on after that. Then I realised that it would turn aside from that path before it reached that milestone. A while later, I saw that it would after all reach that place, but only after a much longer journey - I now have a trilogy mapped out, with my original ending as the ending of the third book. But I also saw that that long voyage would need all its energy, and what I had expected to be only a pause would be the final destination.
Of course, that's only how I see it at the moment. It could be that I am holding my map upside down.
Wherever the story wishes to go, it is wise to follow. Don't ever try to force it anywhere, or else it will become unwilling and refuse to work for you.