There is a difference between saying "I want" and "I prefer." When you say that you "prefer" one thing over another, it implies that the other choice is also okay. If there are two types of cookies - chocolate chip and peanut butter - someone asks you which kind you'd like, and you say that you'd "prefer" peanut butter, that implies that chocolate chip would still be acceptable. But if you say that you "want" peanut butter, that does not necessarily mean that chocolate chip would also be acceptable. I personally do not like chocolate chips at all. If I said that I wanted the peanut butter cookie, it would mean that that was the only cookie I was willing to eat.
When we make choices every day, big and small, most of us have a variety of levels of preferences, from choices where things need to be one way because the alternative is unacceptable, and choices where we don't care one way or the other. I would say that I have pretty strong preferences in most cases. My list of things that are completely unacceptable is higher than most people's lists, and my list of things that I don't care one way or the other about is shorter than most people's lists. I am also not willing to function or be nice to anybody if I am forced to do an unacceptable thing (and that list is very long). These facts make me non-chill.
The reason people who don't know me might see me as chill is this: When you're doing things with other people, you normally try to find things that everyone likes to do, and take turns choosing activities and places to go within the realm of activities that are acceptable to everyone. When I was younger (before high school), I used to be very bossy with my friends and always insist on doing things my way. As I've gotten older (and gotten out of forced environments like college), I've figured out how to manage being picky while also being nice and being a good friend. My strategy is to let other people make choices whenever I feel like I could go either way, so that when I can't go either way, it will be okay for me to make the choice because it will be my turn to choose. For example, if someone asks me, "Do you want to get Indian food or Thai food?" I probably will have a preference. I almost always have a preference, even if it's small. I'm not as indecisive as I might seem. But even if I have a preference, I still like Indian food and Thai food. When both choices are perfectly acceptable to me, even if I prefer one over the other, I figure it's a good time to let other people choose. I do this because I know that the next time a choice comes up, it might not be that way. If the next time we get together, we all decide to order a pizza together, I don't like a lot of pizza toppings, which means that if we're just getting one pizza, the entire group might not be able to get the pizza that they want because I won't eat it. And it's not just about food. It goes for everything. I know myself, I know that there will be lots of times when I'm not willing to do something at all, and I know that these times will come up much more often for me than they will for other people. I feel like the only way things will work out even in the end - that other people won't feel like they're always stuck doing things my way - is if I always let other people choose what they prefer when I would be okay either way. I have to spend a lot of time with someone, do a lot of things together that we both enjoy, and most of all, know that they respect my needs and are not going to try to change me into a more chill person, before I will express a preference in cases where I could go either way.
So if you think I'm chill because I'm always saying that it doesn't matter to me and that whatever other people want to do is fine, that is not the case. I'm saying those things this time because I know that next time, what other people want to do might not be fine with me at all, and I want to make sure that you get a turn to choose.