I wish I knew. I have been a Siamese cat owner since I was about 12. Love the breed. They are amazing, social, talkative cats.
I have to admit, my current meezer is different from the rest. She is aloof, and not very friendly. She hides a lot, and hisses at anyone who finds her. I got her as a kitten, so it isn't nurture. She was a rescue, so I don't know what happened to her before I got her. I do know she had a brother who was "unadoptable" according to the rescue organization.
When I tell others about her (because no chance anyone not in my direct family would actually SEE her. People tell me that is what all Siamese are like....mean, antisocial, hissy. I had no idea people thought this. I blame Disney a bit, with the mean Siamese in Lady and the Tramp.