alone

I've been that friend for the longest time, and let me tell you: it works both ways. If you are a non participant, people will stop asking you out. If you don't share their enthusiasm in conversation, people will start leaving you out. You can't expect people to wait on you hand and foot, you have to take initiative too. For example, when someone invites you to a concert that you don't really want to go to, maybe because you don't like the band or something else entirely personal, you are rejecting that person's proposal. People don't like rejections in general, and thus, to avoid it, will stop asking you, thinking you want to be left alone. Hence the misunderstanding that you're friends.