Lack of confidence and self esteem. When I speak with people (usually because I have to) I start to question what I say as I say it which makes me speak slowly or take long pauses between words. Or in the middle of expressing a thought I just say "whatever" and cut the conversation short.

Personally it all stems from self hate and thinking the person I am speaking too doesn't really care what I have to say and I'm just wasting their time.