I have had some relatively long periods in life where I was much more extroverted. ("Long periods" as in 9 months or maybe a year or two.) But always, always, it reverts back to the need for lots of solitude.
I like people, and like being around them, and I'm friendly and kind and like to make people comfortable. I think this is common amongst INFJ. I'm a "people person", for sure. But... need lots of alone time to recharge. In my alone time I'm thinking about people, or watching movies or reading books that are all about relationships and intense characters, too. It's all people for me, even when I can't be around them for a while.
I think this makes it difficult to understand INFJs, unless you are really aware of what's happening.