Hi. I'm dxed with SPD. I also have some StPD tendencies. I used to feel lonely and alienated when I was younger. It's very rare for me to feel that way now. I used to think I was adopted when I was a kid because of how unattached I felt from my family compared to others. I don't remember ever being able to truly bond with someone else, so I guess I really "don't know what I'm missing".
So, emotionally and intellectually I really don't see the value of relationships beyond the sociological reasonings.

Where being schizoid comes from for me... hard to say. I'll probably never really know. Both nature and nurture are possibilities.