30%: I can't decide what I want to say before the conversation moves on
30%: I am doing something /concentrating on something and I don't want to talk, or I am lost in my own world, very happily, with no forwarding address.
10%: People can't understand what I am saying and make things worse by demanding explanations in the middle of a thought or, very rudely, just walk away as I continue on without them
Its just easier to not talk and not have to be interrogated or ignored.
10%: I can't find an opening in the conversation
10% : I have no idea how to continue the conversation
10%: Someone invariably walks between me and the party I *was* talking to, and takes over the conversation like I am not there. 
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People think there are four elements to the world; fire, wind, water and earth. They are wrong. There is a 5th element - surprise. - paraphrasing of Terry Pratchett "The Truth"