yeah i've experienced this, especially in the past when i was more prone to letting people walk all over me. part of the reason is due to my tendency to be compromising & accommodating. i think people can get used to this aspect about me; that is, they know that they can often manipulate the situation to their benefit, knowing that i won't usually complain. this does not imply lack of awareness of what they're doing on my part
neither does it necessarily imply any maliciousness or desire to screw me over on the other person's part - it's usually just that they are taking care of their own interests... which is what people do. some people may become unpleasantly surprised when i suddenly & unexpectedly refuse to budge on something. i become this way when i lose the desire to be accommodating, perhaps because i feel that the person has gone a little too far, for whatever reason. it could be that they try to get me to sacrifice something i do not want to give up, in order to avoid having to do that themselves. or it may be that they have abused my willingness to go along with what they want (perhaps by asking too much of me), and i have to make it clear that i am not here to continuously give them what they want. etc.