In my opinion, we are nervous becoz we find it hard to creat ideas in brain about the topic we need to discuss, or becoz we don't know how to express our ideas :)

Because they are afraid of making mistakes... they think that if they don't speak the language perfectly, native speakers will think they are stupid. This is why it's better to practise alone at first,and start talking to a native speaker only after gaining some confidence.

I guess, we just fear the wrong pronunciation that might lead to a misunderstanding. ..

I feel nervous talking in Spanish. It is fear of the unknown, fear of mistakes and fear of accidentally insulting someone. It is because as adults we are conditioned to "get things right" especially simple day to day things and what's more basic than talking? To me english is as easy as breathing but Spanish uses so much of my brain I can't seem to remember my own name. If you are speaking a second language you should try to swallow your fear it is not real, people can tell you are learning and will almost always try to help. Speak speak speak and then speak some more it's the only way to improve.

Yea,in my opinion,it is smth like phobia.This people are afraid to speak even their native language with somebody