My ds (10 this week and has AS) constantly asks if I'm upset. In fact it's the first thing he says when he sees me every single morning. He is also convinced that I don't love him as much as I love his siblings and no amount of reassurance works. 
As merlincat said, I think its because he can't read the subtle signals the others pick up on that reassure them that all is well and they are loved. A little smile here and there, ruffle of the hair etc.
Last week I started to wonder if perhaps I am not obvious enough with my emotions eg not smiling enough. When I was younger, I used to get a lot of 'cheery up it may never happen' comments, so I started thinking about whether or not my face is set to default grumpy!
Them when I was having a stressful day, my dd - who is 3, said to me "Mummy what has happened to your lovely smile, where's it gone?" so I guess I do smile enough after all. 
Ds1 also asks me if I'm cross all the time. When he was younger I worried this was because I was pretty short-tempered when I had PND with ds2 and he got used to me always been in a bad mood, but since he got his dx, I've realised it's actually because he genuinely can't tell if I am cross or not from my facial expression, body language and tone of voice.
Merlincat - I know what you mean about the self-fulfilling prophecy. It's kind of inevitable really when they are that incessant.
Ds suffers from extremely high anxiety levels, pretty much across the board and is extremely needy and clingy and in many ways is a lovely, caring boy, but his insecurities often come out in the form of verbal aggression, which he then regrets and as a result gets even more insecure about whether or not he's upset you - its a bit of a viscious circle really.
He's another one that cries easily as well, from regular tears dripping quietly down his face without a lot of fuss, to full on wailing and has had whole days at school where he has basically cried all day.
I'm really worried about secondary school as well, because even if the staff understand and handle it well (which I can't believe will be true of every member of staff) there will be so many more children, who aren't used to him and won't be as tolerant as the classmates he's been with since nursery and he will be wide open to bullying. 