When you lose track of a friend.
I know it's happened to me in the past. You grow close with a friend, they move or something happens, and you lose track of them. No matter how much searching you do on Facebook, you just can't find them. It's one of many lonely feelings you'll have in life.
When you move to a new place.
Many people actually don't experience this one. A remarkably high number of people die within 60 miles of where they're born without ever having moved away. But you begin to experience true loneliness when you move to a new city and you have to find all new friends.
When you lose someone to death.
There's perhaps nothing more sorrowful than losing someone you love to death. It's something we all have to experience at some point in our lives, but when we lose someone before we die ourselves, it's unbelievably tragic. Never another story from them. No more quick cups of coffee. Gone.
When you have a good idea but no one will hear you out.
This one probably happens to you a lot at work, at home, and among friends. It's tough when people not disagree, but just don't hear you out to begin with. It feels very disrespectful and lonely.
When you struggle alone.
It's amazing to have a support structure that can help you through your battles, but sometimes, you struggle with things alone. It's a lonely feeling, but the bright side is that you grow stronger for it.
When someone leaves you.
When your lover leaves you for any reason, there are few feelings more painful and lonely. It becomes even worse when they leave you for someone else, effectively replacing you. But you know what? Only assholes do that kind of thing. You're fine the way you are. You'll find love.
When you face down your worst fears.
You have to do it alone.