I have an old neighbour that I talk to regularly, who has a DS the same age as mine. We regularly meet up and have playdates and coffee etc. I don't know when, but soon after we began talking I found that she can be a little sensitive regarding her son's development, and learned to not talk too much about what my son is doing that her's isn't, so as not to upset her.
We both started potty training recently. My DS must have been really ready because he took to it really well, he was completely dry in five days or so. I was very lucky. We compared notes, as you would. She asked how he was doing the other day and I told her he'd been completely dry at night for a while now, I intentionally didn't tell her until she asked, because I knew she'd be upset about it because her son was still in nappies at night. And she was. "Oh, that's nice for you. Anyway..." 
I just don't get why she has to be this way. Her son is developing normally, he's not behind. My DS does a few things that hers can't yet, granted, but he's a couple of months older, so that could be why. I'm getting so tired of stepping on eggshells with her. She's the only other mum I know of a similar age to me, with the same age DC. She's a lovely person apart from this. WIBU to ask wtf her problem is?