We may once assume that someone might simply be born either an introvert or extrovert, as if these values were biological, like skin and hair color. In fact, it has a logical explanation why someone became an introvert or extrovert, and that is a result of real experiences. It is certain paths that are so distinctive through our lifetime that determine the categories: at which end of social spectrum we fall.
I'm going to propose that we all have the tendency to become naturally introverted, at some point. First and foremost, introverted people are more likely to always be on the look of others powerfully, even before themselves. From a young age, these people would think to themselves: "What do people around me, and the unstable circumstances, want from me?" This contradict the ability that a child should have and grow more, called the freedom. As opposed to ask implicitly: "What do I want? How do I feel?", they had to attune themselves more and more, and too early, to the needs of those surrounding them.
Maybe there was an avoidant mother, or negligent caretaker, who couldn't cope with great demands. This child saw that the people needed to be looked after because, apparently they are still a child themselves deep down. Or perhaps, there was a father with an unstable temper who needed to be watched over carefully, thus creating distances and distractions in order to escape recurrences of inconvenience.
The introverts still has a future, though, despite highly liable to if they succeed brilliantly at another task: overcoming the vigilance of being with other people. They will grown to be experts (re: extroverts), as soon as they work on managing difficult characters and providing themselves with what they actually need. However, this outcome will not come true without a very high cost to their own nature.
Introverts lack of experience in being with others because they're afraid of might not actually meet everyone's expectations. Their history might teach them that the only person who could understand their true self is them. Making relationships are actually hard for introverted people. They may be excellent in raising awareness over something, but they will also be overwhelmed with the condition where binary perspective should be the one fulfilled. It's always them or me; never both.
From this point of view, introverts can start to evolve, that they increase openness between what happens when they are alone and what to give when they are with others. Introvert should ideally understand where their pessimism came from and how it entails to a relationship. The strong desire to be with someone else, only to accommodate them, can be cased slowly because solitude would never be the only route to having an authentic happiness.
We need the time to spend on our own, but it is whom we share our energy with that matters when finding some company. If we always put ourselves second to everyone else we meet, then we couldn't help but automatically satisfy the moods of others and we're going to end up really exhausted. We always crave to be alone because being together still takes us far from who we really are, but that doesn't mean we should bottle everything up on ourselves.