Maybe people don't like feeling like they're being pitied, or patraonized. Whether you intend to or not, it sounds like in your first post that you talked to them BECAUSE they weren't attractive, and felt like you were doing them a favour. Like they should feel grateful to you because of it, and that they should have to work harder to make other people like them for something that ideally shouldn't matter, or is largely subjective anyway...
But that's just how I'm reading it.
I don't have problems with my physical appearance, but my social skills ARE lacking. I've had people walk up and talk to me, and it was OBVIOUS that they were doing it because it was a 'good deed'. Their conversational attempts were completely bland and artificial. Trust me, there's unintentionally frivolous, and then there's complete and utter lack of effort.
I probably should have thanked them for the effort and made some excuse to dismiss them, but I rudely ignored them instead.
However, in the end, I don't feel too bad over it. They put next to no effort to it, and acted completely offended when I reciprocated a similar amount of appreciatation. I don't need people acting like they're doing some kind of huge favour or sacrafice for me, when they're not really doing anything at all.
Edit;
Their attempt at conversation went something like-
"Hi, I noticed you sit here alone a lot. Do you not have any friends or something? We'll sit and talk with you."
Not so eloquently said either. Trust me, I was there, and it was offensive, annoying, and tasteless. Aspies are supposed to end up offending people frequently, but I notice most people don't bother with tact regardless.
It's also wasn't an infrequent gesture. I've wandered off to isolated places to sit and read on many an occasion, and different groups of girls, frequently in 3's, want to walk up and bother me, good intentions or not. The repetition just added to the insult of it though.
The thing is, I did have a couple of friends with shared interests in every school I'd been to, I just like having a lot of 'me' time with a good book, away from the noise of large groups.