The birth of my son was quite traumatic and to be honest, he was a difficult baby up to 18 months (although, of course, very much loved and wanted).
I have a very demanding job as a teacher (and now head of a core subject) and DH also works. He used to teach but gave up when DS was 2 for health reasons. We could afford for him to work for himself, flexibly, which supported his health and means he can do school runs etc for DS.
So, time was also a big factor - we can give ds enough time and attention despite how busy life is in term time.
Money as well- we are very fortunate to be comfortable financially and I wouldn't want to be over stretched. We can afford for ds to do activities etc. We can afford to take him away - we are not extravagant but even things like a nice cottage in the Lake District doesn't always come cheap.
DS is 10 now so we are applying for secondary. Unfortunately, our local state option has gone downhill and I didn't like it at the open eve. We are lucky that we can afford to send ds private but no way could I afford that for 2 or more.
I was one of 3 and although I get on with my brother, my sister and I have never got on and she was very nasty growing up (still is- we don't speak now) so I was conscious of bringing another child in to the mix who may not get along with ds.
We (ds included) love our little unit of 3. We do a lot together, our house is calm and peaceful. I honestly wouldn't want it any other way.