I've decided I'm no longer gonna hold the door open to my apartment building to people I don't know.
Is it for security reasons that you won't do it?
No, the door isn't locked. I just think it's too much of a hassle to hold the door.
Although you obviously don't have to hold the door open for people, it's a nice gesture and common courtesy to do it. If you never hold the door for people, you will get the reputation of being an antisocial and rude person.
It doesn't matter. I want to keep to myself and not deal with people.
There's nothing wrong with wanting to keep to yourself, but you should still try to be courteous to other people. The small gesture of holding the door open for someone can make their day a little better if they are having a bad day. It also makes them more likely to return the favor.