It just reminds you that you are alone.
I don't have an issue with being alone. A lot of times I am quite happy with the fact that I don't have a partner. It's mostly when it's kind of later in the evening and everyone is all into whomever they are with and I'm just there. I have friends, and I love them, but it's not the same thing. Having a strong arm around your shoulder, a shoulder to lean on, a hand to hold. I don't begrudge these people their happiness, I'm happy that my friends are happy. I truly am. It just reminds me that I'm the only one sitting there without that other person to interact with.
Although, I think it's better than hearing someone say, "Don't worry, you'll find this one day." Remarking upon my comment that I don't understand the, "I miss her when she walks out the door" comment. I value my "me" time. That time when I can just sit around and do things that I want to do that are private (like watch really bad romantic comedies). The idea of missing someone that much is foreign to me. Especially when I know that I'll see them in about four or five hours. I have never understood that whole thing.
Maybe it's because I haven't found it yet. Who knows?
It just makes me not want to go out with couples anymore. Especially when they can't keep their lips off each other for more than ten minutes.