This might be caused by a number of psychologically connected reasons. Psychologically, it may be associated with childhood trauma that makes one to have a lot of fear when communicating in public. It is true to say that if all this is not faced and worked on, it keeps elevating with time. As we avoid every opportunity to speak in public, the fear keeps growing gradually. Low self esteem can mostly be the reason as to why some people avoid public speaking or talking to friends. Where a person feels unworthy, unwanted and is less proud of himself or herself, it lowers self confidence towards the individual. However, it may just be someone's personality referred to as introverts. They may hardly speak and they feel okay when they are alone and their socializing skills are very poor. However, personality can change over time especially when that particular person finds someone who is compatible with their interests.
On the same matter, other people are jus naturally confidence and have no fears facing a bigger congregation to talk to them. This people have strong social relationships are able to solve their challenges and face their problems very easily.