Surrounded by a thousand people, I, still feel all alone...

Why does loneliness hit at the most inconvenient times? I'm surrounded by friends, yet sometimes, out of nowhere, loneliness hits harder than anything else. When least expected; it's immobilizing. Like getting punched in the stomach, it's capable of knocking the breath out of you. And the hardest part about being lonely? Not knowing what it is I'm looking for to make the loneliness subside. It catches me so off guard sometimes that I can't even understand why I'm lonely.