Why do I feel like I am always left out even though everyone around me makes me feel like I'm not?

I get what you mean. When I was younger, I often felt left out and later I found out why I often felt that way. Sometimes I still do feel like that even now that I'm older. But I think it's because we are not familiar with whatever everyone is talking about. It could be possible too that we may have missed an event with them that's why your friends are closer with each other than with you. For example, it could be possible that you were not able to go out with them when they went out for a movie. And during that time while you weren't there, they all just grew closer to each other. But that doesn't mean that they don't like you anymore. Sometimes there are just some things that we won't be able to relate with. Spending a healthy amount of time with others can help you resolve your feelings of being left out. Another way you can cope with this is by accepting the fact that everyone is different, has different interests, experiences, and type of relationship with others.