A lot of people mention things like anxiety or fear, but I would argue it's how unpredictable it is, I see that as the root cause of everything else. Imagine for a second you would know that you always stutter on words that start with the letter 's'. And when I say always I mean 100% of the time without exception, but you also stutter at no other letters or words. Finding replacements for words with 's' at the start would be so much easier and you could speak without worrying as long as you avoid that one letter. But that not being possible and every time you speak being a coin toss is the worst part for me.