I am sooooo johnnyoffthespot here, since it seems to have started when I was in hospital. We HS an only, she's awesome, the experience is awesome, and people say awesome too much anymore. DW and I were both the eldest of 2, spaced about 2 years apart in both cases. We didn't start out seeking an only per se, but after a miscarriage, the whole experience with DD has been so smooth from the get go that I find I'm sometimes an advocate for families of three. None of that makes logical sense, but this is a heart vs head thing for me.
I also had terrible rivalry with my sister and we're still estranged, so put that in your Freudian meat grinder as you wish.
Oh, do I wish for more? No.
Is DD happy as an only? She has said so on many occasions, yes.