Why am I so awkward?

Ok so I’m 17. I’m a senior with VERY little friends. I’m not like most of my family or even people I see. Other people have at least 5 close friends and even more friend groups. Me? Uhh. Idk. I’m rather lay in bed and curl up with a good book. Can I go home now? Lol. That’s me in a few sentences. I’m not even weird or hard to approach, I don’t think. It’s just my poor social skills. A few days ago, in AP English Lang, we were asked to turn to the people next to us and talk about our thoughts on the subject. I turned to the only girl next to me. “Umm” I turned to her tentatively. She smiled at me and started talking to me about the subject and what we liked. I couldn’t manage anything but a “yeah” and a closed mouth smile. Once we were done our one-sided conversation, I could clearly see she was uncomfortable with the awkward silence. I flipped through the pages of my textbook pretending to be occupied. Now this would be a regular thing if I didn’t see her earlier in the day start a conversation with a complete stranger. It was smooth and full of laughter and genuine interest. Only in this instance I figured it wasn’t the fact that we don’t know each other whatsoever—but me. WHY can’t I hold a good conversation to save my life??