I suffer from this; at times, to a ridiculous degree. I will go out to eat with a friend, have a craving for a salad, then allow myself to be talked into sharing an order of something I never eat - deep-fried whatever - that I don't want, and won't enjoy.
Why do I do this? With some people, I find it easy to say, "I don't like such-and-such" - those are the people whose response to that statement will be, "Oh, ok." It's the pressurers I have such a hard time resisting. The manipulators.
The ones who make repeated wistful comments about how they just love deep-fried whatever, but a full order is too big, and wouldn't I like to share it with them, I can eat salad anytime, and deep-fried whatever is so good...I'm well aware of the expected response, and I'll start to feel myself caving.
I'll try to withstand it by stating my yearning for a salad several times, but reiterated plaintive remarks chip away at my resolve. I begin rationalising, "Oh it's only this one time, and the other person really wants this, and how important is it, and why be selfish, and it's only one meal..." and before I know it, they are happily eating something dripping with grease, while I eat the tiny bit of vegetable (put on the plate purely as decoration,) and wonder why on earth I agreed to this?
I find it immensely difficult to stand my ground with some people. I can withstand outright bullying with greater ease than soft pleading. Why is this? I believe because bullying demands, and I can see the demand for what it is, and refuse with no feelings of guilt. (Or very few.)
Manipulators plead and wheedle, play on my guilt feelings, and imply, "You can please me, and be a good friend, by giving me what I want." (A friend's young son once offered me the piece of cheese from his sandwich, with the remark, "I think I'd like you, if you eat this for me." The same basic message, just more upfront.)
What is this really kind of self-abnegation really about? Why would I agree to eat something I don't like? I believe it has to do with my ego, and the other person offering me two reflections of myself, one is generous and self-sacrificing, and a good friend, the other is selfish and greedy, and a bad friend. (I'm exaggerating for effect here, but this is how manipulation works.)
If I am having a good day, I can detach from either of those choices, and choose my own self-image. Days like that, I'm impervious to manipulation, because I recognise it for what it is. On my bad days, when I'm in HALT, or not feeling well, or out-of-sorts for whatever reason, my self-image tends to be a bit shakier, and my desire for approval is much stronger.
12-step helps me to be willing to investigate these aspects of my character, share them with another person, and with any luck, have a good laugh about it.
I can use my sponsor's reply for most requests - "I'll have to get back to you on that." It gives me breathing room, without someone's hopeful gaze affecting my reasoning process. But that doesn't work very well in a restaurant situation, so what are my choices?
I can be aware that this person is someone who is skilled at manipulating me. Call them on it. If I'm not comfortable doing that - pay attention; try to stand my ground, and ignore the clamorings of my ego. If I cave, accept my frailties, and try for a better result next time. That's all I can do, and that's enough.