When I was younger, I used to fit the description of an introvert precisely. I found myself drained in social situations and people would frequently ask me if I was "OK" and why I was so quiet. I was the silent, quiet guy who hadn't even kissed a girl through high school but who had no difficulty being top of the class without studying or paying attention during lessons (i.e. I wasn't retarded). On the MBTI in my first year of university, I scored very high on "I".
As I grew older I started realising that many of my "introverted" behaviours were making it difficult for me to have a fulfilling, healthy life. As I started exploring the problem space, I learned I suffer from a lot of social anxiety and self-esteem issues. I realised, for instance, that is wasn't people I found draining in social situations, it was the constant feeling of being "different" and not understanding social interactions, as well as taking social gaffs personally as though I were flawed as a person. Feeling inadequate used to stress me out enormously (and it's still something I suffer from) and drive me to avoid social interaction.
As I've begun tackling these things over the years, nobody tells me any more that I need to relax or that I'm too serious (although I do find I have distinct 'modes' where if I'm in work mode I just won't get somebody joking around with me, for instance). At times I'm excited to go out (with or without friends), and other times I am still reluctant to go out, i.e. I've learned to enjoy socialising but I'm not a social butterfly. I have become more interested in small talk with random people after experiencing the positive upside of meeting interesting randoms, but I still can't stand pointless small talk in the office (when, for instance, somebody asks about my previous evening rather than just getting to the point about why they walked over to my desk). All of this makes me skeptical when I see "introverts" resort to "extroverts just don't get it" rather than explaining what there is to get -- I'm not convinced that the "introvert" has explored the possibility of social anxiety or depression driving their behaviour, the symptoms of which were largely invisible to myself while I was in the midst of dysthymia.
In the past 2 years I did the MBTI twice as part of training separated by a year and in both cases I scored 50/50 on I and E. I've never seen anybody talk about shifts in MBTI, but it certainly does happen and makes me skeptical about articles such as these which treat I/E as all-or-nothing traits (or which pay lip service to a gradient of introverted/extroverted personalities, but don't discuss important matters such as social anxiety and low self-esteem as being a source of behaviour). Finally, I've noticed that in social situations I tend to be more "I" but in intimate relationships I'm very "E", which is something I've also never seen really discussed.