It is very lonely and crushing. You begin to obsess about the fact that you have no friends and wonder why. You turn your harshest critical eye inward and self-examine and magnify all flaws, searching for a possible answer. Since you obsess about it so much, you begin to feel like everybody else must notice what a loner you are too. Then you become paranoid, feeling like whenever other people look at you, they see a friendless weirdo loser with no prospects in life--no future. This only makes it that much harder to make new friends. So a downward negative spiral begins--being slightly shy turns people away, making it more difficult to make new friends, which in turn makes you less socially capable. It's next to impossible to enjoy yourself at parties, on the rare occasions that you get invited to one.
While you're walking alone down a street, it feels as though all passers-by are higher order, socially intelligent, fashionable people who effortlessly glide through life on a winged chariot of charm and charisma. In comparison you are alone and defective, and very bitter about it. You begin to hate; nay, to despise the "popular" people in all their blithe and breezy self-confidence.
That is what it is like to have no friends. Only until someone comes along to pull you out of your self-imposed seclusion, will you begin to return to normalcy.