One of the symptom of being sick is feeling like you have to isolate yourself. Like for example when you have the flu you don't really feel like going out and spending time with other people. probably the thing that you would prefer to do the most is spend all day in bed by yourself.
Pain is depersonalizing, when you are experiencing pain you are really not being yourself it changes the way you interact with others.
Suffering from a mental illness is like having a flu for decades of your life. Your personality is being altered by the pain for years and years. Only that nobody sees what you are experiencing emotionally, so they blame your unusual behavior on your genetics.
It doesn't make sense to say that this is natural, because we need people, as we are social animals it just goes against self preservation rules to be born with a reduced desire to interact with others. Is like saying that someone was born with a reduce desire to eat. If you are afraid of food, in order to develop a stress reaction to it you need to experience some negative experience with it first. Like you might be afraid of peanuts because you are allergic and they almost killed you once.
Your nervous system has to perform two main tasks. First it has to make sure that you avoid all threats and possible threats that could lead to an early death, in neuroscience this is called the sympathetic nervous system. Second it has to solve problems in regard to your needs of eating and reproducing, this part of your nervous system is called the parasympathetic nervous system.
When you are under constant threat, you become predominantly controlled by your sympathetic nervous system and one of the symptom of to tun off social engagement as tool of preventing threat.
It is not a problem of overcoming shyness, because shyness is a symptom of a bigger problem and symptoms can't be treated at best you can get some relief. The shyness will go away by itself after you address the cause.
Until you won't address the reason why your body perceive danger often enough that it keeps you in a predominantly sympathetic nervous response your social life is going to suffer.
There are two scenarios in which a person gets stucked in to chronic pain and stress. It is either constant re injury, experience abuse to frequent enough that doesn't gives your body a chance to heal, or it is an adaptation of the brain where the brain becomes hypersensitive as an attempt in avoiding future re injury.