Because it is kind of a downward spiral really. I stutter too and used to do this too, but it won't “heal” or “resolve” anything. You are just going around the problem. You are going to get scared of certain words, which will only make the situation worse. You need to handle the source, otherwise this will happen to more words and speaking will eventually get even more difficult. Try to find which sounds you are having a hard time with pronouncing and if it is at the start of the word, or more at the end. For the periods of stuttering thing, for me it has to do a lot with scheduling, stress and my mood. Try to regulate your life a bit more or smthing like that. By any means im not a professional, so if any of this doesnt apply to you or doesnt help, throw it in the garbage bin.