I don't understand why I feel so bad

It just doesn't make any sense to me, I have these great friends and great family (most of them). We throw parties, we go on vacations, and I keep my schedule slammed with stuff to do most of the time. I guess my real question is why do I feel so unsatisfied and lonely all of the time? I just had a pretty good day, it would rank a 9 in my book and yet here I sit feeling just horrible and all alone in this world. It doesn't make sense and I've tried to shake these feelings off for the past couple of weeks now, but I am simply running out of solutions. I don't like feeling like this, mostly because I am feeling completely vulnerable and insecure. So I am just trying to figure all of this out.