Not to give pushback on some of the hate here (even though whenever someone says, "Not to ___" it virtually always means they are), but as far as the withdrawing thing and door-slamming I just have to put my two cents in.
I have been unfortunate to have met some extremely toxic personalities in my life. NPD people, clingy people, insulting people, heartless people, you name it I've probably known them. I used to let everyone in because I just wanted to be close with people. Unfortunately, I learned very early on that letting the majority of people I met into my daily life yielded some extremely bad results. At first I thought it was just a one-case scenario, but I began to realize that many people could be horrible to have in your daily life through repeated experiences. Of course, this is not everyone. There are many wonderful people out there who I would love to have in my life. Unfortunately, it's hard for me to tell who's who sometimes. So, my natural instinct is to initially trust no one. I've developed a hard outer shell because in essence, I have to. If I don't people will try to walk all over me, and I know that from personal experience. I don't want to not trust people and keep them at a distance, but I feel, at least for me that it is a necessity especially in today's society. That is, until they've proven they're trustworthy.
The way I make my evaluations of who is trustworthy and who isn't is pretty simple. I just judge them by their actions, and MUCH more importantly, the thoughts, feelings and motives BEHIND their actions. I am an excellent judge of these things. If someone is doing things that are wrong, or is a person I want to stay away from, I pick it up extremely quickly. They can put on a fascade if they want, but in most cases, I will see right through it.
Whether this is why all INFJs are the way they are I can't say. I'm just one. But I stand firmly by that feeling that it's OKAY to shut people out of your life if it's the best decision for you. To me, that's common sense. So, people who stay in abusive relationships, or trust random people they don't know are somewhat lacking in common sense to me.
As far as introverts wanting alone time, pretty much every introverted type is like that. That's just the way it is for us.
Lastly I'll address some of my weaknesses that I don't like about myself:
-Slow at taking care of daily errands/spends too much time sitting around contemplating life.
-A desire to be comfortable all of the time.
-Overly-perfectionistic (although sometimes I just want things to go well really badly which I think is human).
-Awkward with people/hold back part of myself out of shyness.
