When you read alone, your brain isn’t under social pressure, isn’t over-monitoring itself, and isn’t juggling unpredictable feedback — so speech becomes automatic, rhythmic, and fluent. No audience = no threat = no interference in speech motor timing. You’re much more relaxed. When you speak to someone, adrenaline and dopamine kicks in and this raises anxiety for someone who stutters, affecting your breathing and heart rate and tenses up your vocal muscles etc. It’s a vicious cycle.