Softly has it right, and totally described me when DS was younger
I actually felt about DS self weaning in case I had "encouraged" it. He was FOUR. There wouldn't have been any problem if I had. It took about a year for me to stop feeling guilty and fretting about it, far longer than it took for him to stop mentioning it.
I also beat myself up for ages about not doing EC properly because as soon as DS could crawl he was off doing that and didn't want to sit still. Now I can see I was crazy and it was a good thing he wanted to explore!
I was under a lot of pressure when he was born - I was only 20 and I felt people were judging my ability straight off the bat because of my age. I had an unsupportive partner who was not really interested in the baby and whose views on child rearing were practically 1950s. His whole family were similar so I tended to feel a bit panicky about my opinions/instincts on how things should be done because I felt I wanted to pick DS up all the time and was happy to breastfeed whenever and they all thought I was mad, I felt like I was having to both prove my way worked and fend off criticism all the time as well as being constantly vigilant in case they were letting him cry or giving him formula, both of which would have really upset me at the time. (And they did try to give him custard when he was about 2 months old!) Add to this my own mum was very pushed out by XP in the early days and I ended up on various forums for advice and most forums I found 5 years ago were American ones, and in the US it's very much more split as far as I can gather, there's no in-between, do what you want, throw the book out of the window kind of muddling along (well I'm sure there is, but they aren't hanging out on the internet obsessing about it!)
Anyway, I suppose I got myself into this mindset, that you were either co-sleeping, breastfeeding, sling-using, gentle discipline-ing, etc, or you couldn't wait to get your body back, all the drugs in birth, cot in own room doing controlled crying as soon as somebody said it was okay, never listening to your child, disciplining solely with reward charts and the naughty step, and so everyone was either "us" or "them". I spent my time whenever I met new parents hoping desperately that they were an "us" although then I'd try to convince them of all the things that they could be doing better if only they knew about them
convinced that every slightly different choice was a matter of either not knowing that something was possible or being brainwashed by "society" to think they shouldn't. I was such a nob, it's a wonder I ever managed to make any friends at all. I was terrified of the "them" parents and would avoid them or try to hide my parenting as much as possible because I was scared they would criticise me. 
It's totally nuts - I can see that now really clearly, and thanks to mumsnet - really, ONLY thanks to mumsnet, I have realised that the "us and them" theory is totally bonkers, most people are muddling along somewhere in the middle, the idea I had of the "them" parent is a caricature and not true (possibly similar to the caricature idea some people have about AP parents...) and there's nothing wrong with offering information when someone is asking for it but most people do already know what the different options are and don't need to be told. But I was still beyond shocked when I met someone who did half of the AP things but then smacked! It totally threw me - I thought, that doesn't go together! And this was after I thought I'd thrown away those old stereotypes.
So I think the criticism comes from a combination of anxiety in what you're doing yourself (I remember a really heartfelt decision NOT to use a rear facing car seat when DS needed to go into group 1 because I was just so tired of feeling I had to defend and explain my choices all the time and I couldn't bear one more comment about "Oh that's weird!"), a really warped view of what "other parents" are doing and the reasons why they are doing it and hence a genuine feeling that the actions of other people are harmful.
It definitely abates as your child gets older which is why most people in those groups are parent of one child, under 3 years old. You do get the occasional older child's parent or parent of more than one child but this is either down to extreme stress/anxiety or they can see through that stuff but are a bloody saint ignoring/trying to bring some rationality to the more judgemental discussions.