1) How do you express your likes and dislikes connected with people and how they behave?
If I know someone well, I will tell them directly when I am bothered by their behavior, or at least I will ask for a change. If I don't know someone well, I will tell them directly, only if I am directly impacted by it, for instance a coworker who does something that makes it difficult for me to do my job. For the rest of the world, if I don't like how someone behaves, I just avoid and ignore them. It is not my job to correct people, or even to expect they satisfy my standards. In all cases, I try to model the behavior I would prefer.
2) Should I believe you when you say something is neutral to you? Especially if your behavior says otherwise?
You should always believe first, foremost, and almost exclusively the words out of my mouth (or writing from my hands). The idea that you would give any precedence to bodily reactions that I might not always control, over the deliberate, reasoned response I formulate is almost insulting, and likely to lead to misunderstanding. I have been told my nonverbal cues are few and hard to read, which suits me fine. I will tell you what I want you to know, and if I err in assessing or expressing my own wants, I willingly abide by the consequences. Many small things really are neutral to me: what to have for dinner, whether to see movie X or Y. If I have a real preference, I will say so. I may also be neutral on much bigger issues when I have not considered them sufficiently to have a sound opinion. I will explain this if asked.
3) I noticed that in any relationship like friendship for example you need to feel welcomed (that your presence and advice are appreciated and wanted). When you use the phrase "I don't mind" is it closer to "I don't care" or to "I like it". "I don't mind" doesn't mean I like it. It is closer to, "it wouldn't bother me". You are right about needing to feel wanted in relationships. I take a very live-and-let-live approach to personal interactions, and see no need to impose myself where I am not wanted, or to hang around people whose company is not enjoyable to me. A relationship should satisfy all involved.