Fascinating thread. My experiences have not been nearly so bad as other people's, but fall into a similar vein, I think. I started working at a university office as an undergrad, and I quickly discovered there were a couple of people there who thought themselves kind of special and wanted to boss me and the other students around in an unreasonable way. It was weird because I was very much the "good girl" and the other student, Ellie, was more of the "rebel" type, but it was Ellie who got sucked into doing extra stuff for these people--like they would call her at home on a Sunday to fetch research articles for them, when we were strictly Mon-Fri, hours at the office only. If anyone had pulled that on me I would have said, "Uh, no."
I remember once I was in the middle of doing something that couldn't be stopped once I'd started, and this person came up to me and wanted me to go to the library and find a certain article for her, in time for her to read it by the 1pm staff meeting. And it was now, like, 12:30pm. I said, "Well, when I finish here, I can get the article for you, but I don't think it will be by 1pm." I can see the look on her face now, like I was being so unreasonable and difficult, when--even if I had dropped what I was doing right then and done her thing--I probably wouldn't have been done in time. I wasn't thinking about setting boundaries or taking a stand or anything, I was just thinking that, I was in the middle of this thing and I couldn't do anything else until it was done, so--not happening.
So she learned pretty quickly that I wasn't someone she could push around, and we actually had a decent relationship. But every time we got new students, she would look for the biggest pushover and set them to work for her. You could see her testing them all to see who was the most malleable. And I don't necessarily blame the students, it can be hard to know what's reasonable and what isn't, and even though our boss didn't like she wouldn't confront the person, so...
On the other hand I do know a couple people who practically throw themselves at people's feet, begging to be used. My friend/former co-worker Emma is like this. We have a mutual acquaintance, Grace, who I guess means well but is incredibly blunt and judgmental; I make only small talk with Grace, because I know she will take any tiny bit of personal information as an invitation to comment on and judge my entire life. Well, Emma discusses personal problems with Grace and asks her for help all the time, even though Emma has come to me in tears multiple times due to the mean things Grace has said to her. Stop talking to her, then!
Emma and another friend, Amy, also have really high standards for friendship (or at least that's how they describe it). They want to be the type of person who drops everything to help out a friend. And then they expect other people to act the same way back to them. Sometimes they run into actual users who take advantage of this, other times they run into ordinary people who just aren't prepared to go from acquaintance to BFF in an instant. I think one time Amy gave an acquaintance a ride home when the person's car wouldn't start, even though it was completely out of Amy's way, and the person was grateful and offered her gas money (which Amy refused); and then Amy was mad because the person wouldn't drop everything to run an errand for Amy later. She barely knows you! I'm sure she was grateful for the ride, but she wasn't thinking it was the beginning of a grand and glorious friendship, you know?