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.