There are several reasons why this happens. Most people feel nervous in the moment that they are trying to speak with another person. This distracts us and causes us to stutter. Also, to speak we use different parts of the brain (as well as muscles in the mouth and face). All of this requires many things to happen at once. We are thinking about what we want to say; forming the words with our mouths; being aware that we are speaking another language which may not feel natural for us. We may be nervous or worried about our pronunciation or the other person's response.
The solution is practice and confidence building. The more you practice thinking and speaking, the more it will become automatic. You will no longer have to concentrate so much on what you want to say. You will just speak.
