Most of the time, it’s believing lies. 

Lies like:
- The world is ending, there’s no hope, and it’s cruel to bring a child into this. 
- The world is ending and it’s the breeders’ fault; having children increases the load on natural resources unsustainably. 
- Having children / getting married means your life is over, you can never do things that matter to you again. 
- No responsibilities and complete spontaneity is the best way to live. 
- You have to have everything all figured out before you have a child. 

I think the second biggest factor is trouble finding a suitable partner.