My sister went mute for a few years after we went on a vacation when she was 2 and left her with a relative, apparently that was extremely traumatizing for her. Now she in her 20's can speak very well but is extremely quite around people she doesn't know. I believe her husband is selective mute and does best when people are direct with him, for a while he didn't talk at all, even if you looked right at him and asked him a question.
I can get very choked up for words when I'm stressed or having a meltdown and it's usually at that time I'll choose not to speak. It's very frustrating not being able to articulate your thoughts and emotions, I feel bad for people who have it even harder 