I've never wondered about other people's choices. I've just thought some people choose to, and others choose not to or are unable.
I'm happily married, financially stable etc but have never felt the desire to have children. After I got married, I was asked constantly when we were having children, but after 15+ years together that has now stopped. I think a lot of people think we are unable, rather than choosing not to - based on some pitying looks I've received when I've said we don't have children. I've stopped saying we are childfree by choice now as it's no-one else's business, and I've had lots of negative comments about it - you're selfish, going to end up a mad old cat lady etc.
I think there is a very strong societal expectation of marriage then kids. Whereas for me, I loved my husband so got married. I saw choosing to have children or not as a totally separate decision from the one to get married (I'd obviously had prior discussions with my husband). According to my mother, this is not "normal". 
The biological clock hormone urge has never happened to me, have never felt broody etc and I'm nearly 40. I do sometimes wonder if it will ever hit me, and am also curious about what it feels like e.g. does it just come over you one day and you just know you want children?
As it hasn't happened to me yet, I think it's unlikely that it will. However I do also sometimes wonder if I will suddenly get the urge, and then it will be too late and I'll regret not having one. At the moment though, I'm perfectly content not to have any. Will look forward to getting a cat in the future 