That is the story of my life right there!
If I'm in a conversation about things I'm really interested in I'm amazing, I talk the most and say everything with excitement and expression and I often have to stop myself from lecturing them about my interest. But as soon as the topic changes I go quiet again, I also get really annoyed when the topic is changed because I was having fun talking about what I'm obsessed with and I want to say more things, and sometimes I'll even revert to the old topic briefly just to make one more point. And like L_G I will just go "yep. yep. right" when they talk about what they like but I have nothing to add and the conversation becomes harder to carry on. Or I'm just not interested at all and I'm waiting for them to shut up so I can start on one of my interests again
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I love to listen and observe people's conversations and do so frequently but I rarely take part in them because I have NOTHING TO ADD even if I tried. My good friends are of course an exception to this rule and I can take part in conversations about what they like... as long as my interests are mentioned as well of course
I also hate it when that happens. I just want to leave the conversation when they get boring but that's rude so I stay a few minutes longer and then leave. If I have something with me, I do that and maybe another interesting topic might come up.