Why was Melissa Sue Anderson so apparently distant/aloof?

Ok, I've read Alison Arngrim's book, (didn't know her brother did that to her, or why he wasn't locked up), am about to read Melissa Gilbert's book.

I can't seem to get a take on why MSA was so apparently disliked/different than the other actors. Was it something in her childhood?

I have read all the books you have mentioned and my take is that almost like Mary Ingalls, Melissa S. Anderson was a studious girl. She was a bit reserved, and very focused on her role as well as her education. If she seemed disliked or different it was probably just because she was misunderstood and/or the other kids didn't have the chance to really know her .
She nailed her role as Mary and I think she has nailed her role as wife and Mom. She knew she wanted to have her focus on her own family. I think she has also represented a very noble standing with how Little House cast should carry themselves. Very proud of her.

No adult ever said anything negative about MSA. I think like with schools, workplaces, etc., the set had its cliques and gossip and groups and she didn't fit in with their behavior at the time. Being "different" is always looked at negatively by others many times. I think the fact it was still brought up was mainly to sell the books. I'm sure without being a fly on the wall, they were told, the more inside gossip, the more discord talked about, the juicier it would be. I don't think Michael Landon would have liked it.

Melissa Sue added a lot to the show, her character was central at times and she did a fine job as Mary. Many older actors gave her a lot of kudos and even if her talent was mainly as a child actor, she handled herself well and I'm glad she didn't stoop as low in her rebuttals.

Gin wrote:

I have read all the books you have mentioned and my take is that almost like Mary Ingalls, Melissa S. Anderson was a studious girl.  She was a bit reserved, and very focused on her role as well as her education.  If she seemed disliked or different it was probably just because she was misunderstood and/or the other kids didn't have the chance to really know her .  
   She nailed her role as Mary and I think she has nailed her role as wife and Mom.  She knew she wanted to have her focus on her own family.  I think she has also represented a very noble standing with how Little House cast should carry themselves.  Very proud of her.

Nicely stated Gin!