@walid373 I feel so complete with my daughter (15 months), my partner and our dog. I will say I do think about "what if" all the time regarding adding on to our family, but it just isn't really a good option for us.
The postpartum period was so mentally and emotionally challenging for me. PPD/A was rough, and I don't want to go through that again when my daughter is old enough to witness me going through all of that... There were a lot of dark moments for me that I'm thankful she won't remember... My physical recovery was also pretty rough.
We send our daughter to childcare three days a week, and we would never be able to afford to send two kiddos to childcare. I would be forced to be a work or stay at home mom, which would ruin my mental health (I WFH with my daughter two days a week and it's a challenge), and really hurt our financial stability.
I really don't want to be outnumbered on the days I have to venture into the world when my partner is at work. Handling my one child is tough as it is. I am in awe of parents who can wrangle two+ kids alone. Incredible...and not for me. lol
TL;DR: OAD because of mental health, finances, and not wanting to be outnumbered.