Because you tense up when you're being observed by other people who may be judging you and what you're telling them. You tense up because of this pressure, and I'd assume because of this you put more force into the movements that are necessary for speech, and also your heart rate probably goes up, which makes breathing harder. Solution: This sucks, but as much as you hate it, the only way to overcome this problem is normalizing talking to other people, or in tense and pressure inducing situations. You have to start from a low level of scary-ness, and move up to higher levels gradually. Talk to one of your friends or family members once or twice a day on a regular basis about a subject that you're both interested in, and watch what happens when you're speaking to them. Notice your heart rate, the tense-ness of your muscles, the rate of your breathing, weather you keep eye contact when you stutter, and stuff like that. At the same time, when you're talking to them, actively try to do something that would normally embarrass you (you can talk to them beforehand and tell them that you're gonna do things like this), for example, talking in a slow and prolonged manner, faking a few stutter, or other silly stuff that would frustrate you or embarrass you in a normal situation. Do this regularly enough, and it should decrease the pressure that you feel that makes you mess up when you're in an important situation. And also of course, gradually move up to higher levels of scary-ness. So for example, you could start asking for a number on the phone from an stranger while faking an stutter or talking slowly, then move up to asking a stranger in person and doing the same thing, then asking a waiter in the same way, etc. Then again tho, I'm not your therapist or speech therapist, and I'm not an all knowing being. This is just what I think would perhaps improve the situation a little bit. Trying it out and seeing whether it works and makes it better or not doesn't hurt you (too much), right? I'm an stutterer who's trying to get better recently btw.