I'm not really sure wether to call this a problem or a habit or something else entirely. In any case, I don't react very strongly to most situations. Starting off small: I really love roller coasters and I'll smile and maybe laugh for the first third of the ride and then I'll just go back to a pretty neutral expression. This isn't that big a deal, but the problem comes when I'm put in a usually sad situation. For example: my friend's relative died recently and he/she told me about it. I was sad (I had only met the person before once, but my friend was sad so that made me sad), but I didn't really show any reaction until I realized that I might have been appearing insensitive. My friend didn't seem to notice but I still want to make sure to avoid offending anyone. Even when I smile or laugh usually it's fake but it's become kind of a natural reaction at this point. I still have trouble in depressing or negative situations though. I just want to restate: it's not that I don't feel anything, I just don't automatically show what I feel. Does anyone have tips for making these reactions more of a habit? It's been making me more and more reluctant to get into social situations. Also has anyone experienced this before?

Yeah, I don't know if it's a common thing though. It's just the way some people are, you sound a little bit like me, sometimes I kinda act to react as I think I should at that time. You may feel sad or happy but you don't know how to express it in a natural way as most people do. Don't worry about it, people that know you will be able to tell how you feel even if there not tears in your eyes or you don't have a smile that pretty much covers your whole face.
Maybe you just feel out of place in that kind of situation? I'm just 17 so most likely I'm not the best to give advice about this sort of thing. Find someone you really trust and talk about it, that won't change the way you behave, but it will help you being okay with who you are.

I have the same problem, well I don't really see it as a problem. I think it is just that we, well at least I, don't stay fixed on one thing for too long, like emotions. Some things just don't hold as much importance to me as other things. There is nothing wrong with it, it is just another way to think.