I think for me it's mainly about trust and feeling like I'm the same as the other person. If I trust you and I feel like we're not from different worlds, I'll open up a completely different side of me, to you. The problem obviously is that the only person I naturally felt this type of connection with was my mother, then later my brother and over many years my sister and father. For everyone else I really don't feel this type of unity, if I can use that word. I feel like we're not the same species and our understanding of one another is thus limited or made void.
That's why I take on another persona when I'm around people I'm not comfortable with. My immediate reaction is to flee and avoid feeling uncomfortable, or for my "uniqueness" to not be exposed. This means I take on a very shy, awkward personality which I can't seem to really change. I think for me it all boils down to a lack of commonality with others. One of the reasons I feel "what's the point of trying?" is because I assume others have this same perception about me - that I'm too different to be associated with. Being a sociable being, with people I don't feel comfortable around, is for me a great struggle.
I still ponder as to why I feel detached from everyone else. If I could figure that out, perhaps my uneasiness around people would lessen, and then I could show them a much more expressive side of myself. That's why I am alone. It's just too much of a mental effort to be with others.