Friendship is relative. Loneliness is absolute.
It's frightening how such a thing can sneak up on you. I woke up in the middle of the night, my sleep pattern no doubt torn to pieces by my unhealthy habits, and loneliness itself stood at my bedpost, staring me in the eyes. Do not misinterpret me - Loneliness is not about being on one's own. I can handle being all by myself - I even enjoy it a lot more than other people I've met.
This is not the same thing as loneliness. Loneliness is a cold, strange realisation that creeps upon one when it's not expected, the weird realization that, despite ones' best efforts, we are always vulnerable to ourselves. Think about that last sentence. You are always vulnerable to yourself.
Therein, I think, lies the real reason to fear loneliness.