Always been this way ... [I'm a Loner Most of the Time]

Ever since I was a child I could quite easily entertain myself and have fun with my imagination. I didn't and I still don't enjoy mixing with other people very much. I find it difficult and very taxing mentally and it can cause me to feel fairly distressed. It's nothing personal against other people, I just find it very difficult. Members of my family don't understand it, they are all very outgoing and I'm the complete opposite. They often tell me that I need to get out more and meet new people, but it's just not who I am. There are a lot of different types of people in the world and it's not so hard to understand that there are people that don't need constant human interactions to get through life and are happily self contained individuals. If I can understand that there are people in the world that need human interactions on a daily basis then why is it so difficult for them to realise that there are people that are the complete opposites?
I have a very small group of friends that I have known for many years and they understand. They know that they will not see me for a week, fortnight or a month until I feel as though I'm ready to go outside and mingle for a little bit. Other than that the only other time I mix is at work where I spend several hours listening to other people's problems. I don't mind that though because it's strictly professional and I don't have to bring it home with me.