Being completely honest. Because everyone else was and I was approaching 40 and thought I would regret not having at least one.
However the biggest reason for me trying to conceive was that DH (then DP) wanted a child. I think if I had had a different partner who didn't want children, I wouldn't have had one.
I was never broody, though I was v. good with other people's children and liked them and found them interesting and funny (especially the naughty ones. 
)
I also had a very close relationship with my niece and nephew and put masses of effort with them in the 15 years after my sister died when they were very young. (Spent almost every moment of leave from work with them for years and helped my parents look after them when their dad worked)
One of my friends with children at the time said she always forgot I didn't have children as I was always happy to talk about children and would like being around them.
I did think having a child would be very hard work and knew deep down I wouldn't make a brilliant mum and despite loving DS very much, I was right about all of that. Luckily DS has one patient, unconditionally loving parent in DH.
I am generally too quick to get irritated, too much of a control freak, too selfish and not patient enough to be a great parent. I am not awful (I hope) but it really does not come naturally to me.
On the other hand I am less of all of those things nowadays. Whether that is age or having DS I don't know but I can't imagine not having DS now.