Why am I always just an acquaintance?

In the twenty short years I've been alive, I've always felt as though I had many acquaintances, but little to no actual friends. That is to say, I've always felt there was a distance between me and the friends I see everyday. I would describe this distance as the kind of relationship one might have with their next-door neighbor as opposed to the kind of relationship one might have with a close friend, girlfriend/boyfriend, or spouse. For example... a neighbor is somebody you see day to day and say "hi" to. You always refer to him or her as "Mr. or Mrs. so-and-so" even if you've known him or her for many many years. You might now and then engage in trivial conversation with them about how nice the weather is, something in the news, the birthday party you just went to, or the rising price of gasoline. You are (hopefully) civil towards this neighbor, but you are usually never close to him or her. The neighbor is not the kind of person you invite everywhere or confide your secrets to. On the other hand (at least by my definition), a true friend makes you feel included. You feel like you share something with your friend. There may be distance, but it is not profound. You're free to joke and laugh about things, and you know each other. A friend is someone whom you can confide in, and who feels free to confide in you. A true friend doesn't neglect to invite you into their home, nor their heart. That is my definition of friendship. Unfortunately, by that definition, I have no friends. I know many people in day to day life who I call my friends. These people also call me their friend. But there's always that distance. No matter how long I've known them, I feel like I'm always the neighbor they call "Mr." regardless of whether they might call me by my first name or a nickname. They don't invite me anywhere, yet they invite their other friends. There is an awkwardness, as if they view me in a different light and can't talk to me about certain things. What I want to know, is why am I always just the acquaintance or the "neighbor?" I've accused myself of every fault. Am I too immature, or am I too old-fashioned? Am I too radical, or is it that I'm too closed minded? Am I ugly? Am I too different? Am I too shy, or am I trying to hard? Why won't they invite me into their hearts?