You go to school or to work. Most likely no one notices your presence, or someone might say hi, but that's all they say before they go back to talking to their friends. You sit down alone, and prepare for another day where you will barely use your voice. Maybe someone will strike up a conversation with you, but most likely they won't. They have friends to talk to, after all, they don't need to talk to you. Most likely you'll go through the day without having to say a word.
Maybe you're the kind of person who doesn't mind being alone from time to time, but being alone 24/7 is a bit much even for you. Are you bitter? Perhaps you are, or perhaps you're just a bit sad. You don't necessarily resent people for not talking to you, after all you can't demand that someone notice you're there, but you still wish they did.
You hear your classmates or your work collegues talking about things they plan to do together. Parties you're not invited to, get-togethers you're not invited to, all sorts of