I haven't been in the middle of a depression while being in a relationship at the same time, but I will take a stab at answering anyway.

Yes, we realize that we're pushing people away and hurting them. And it makes us feel even worse. There's this odd mix of wanting to be honest with them and knowing that the way you feel is going to hurt them more, and wanting to pretend that things are fine even though you know they'll see through it and it means you're lying to them. Many of us who are depressed will feel like we are going to be abandoned and left alone and rejected anyway... especially when the depression intensifies... so even though that terrifies us, we'll retaliate and start to do it to ourselves. Sometimes it's pretty much impossible to believe that someone actually will stay around no matter what, and we are just waiting for them to have had enough and leave.

And those people who don't know us too well? Well, they never do get to know us well. Because we go out and we lie. It's easier to lie to strangers because they won't see through any of the BS. They won't have any expectations of us, and they won't question us about how we are behaving, and they won't be upset if they don't see us again and so we only have to pretend once. We don't worry about them rejecting us because we will be rejecting them at the end of the time out anyway. The only people who get to see the pain are the people that we care about and trust the most - because with those people, we're trying to not lie. We're trying to be honest.

We don't forget about other people, and we do realize that we're pulling away, and that it will make the other person lonely. However, sometimes we literally do not have the energy to go near someone, and will feel pressured too much to meet whatever the other person would normally expect of us. So we might withdraw in an attempt to alleviate that stress for ourself... but also because we don't want to upset and hurt that person even more... and yes, we understand how that doesn't make sense to other people.

But yes, we know what it does. And that hurts us even more than it does the other person, because we know that our brains are the cause of it, and we don't choose it and don't know how to get out of it all the time, and yet our brains spend all the time in the world convincing us of all sorts of horrible things.