Like any complex phenomena declining fertility has multiple causes. The importance of each cause depends on the local conditions.
1. Easy access to contraceptives (they are easily available throughout the developed world)
2. Positive view of abortion (the christian world used to see abortion as a crime, now with increased secularisation more is placed on the mother's preference than on the fetus).
3. Pressure on resources (when having children becomes a choice, it doesn't happen 'naturally', people are less likely to bring a child into the world if they cannot raise it properly).
4. Education and career orientation of women (Educated women make less children, they finish school around 24, start their career and if they don't secure a partner by 30-32 their fertility windows has closed - even if biologically they could bear children, they struggle finding a partner to raise children with).
5. The marriage institution has collapsed (Marriage is the bedrock of children rearing - high divorce rates, divorce courts, normalisation and acceptance of divorce has made people less likely to marry and thus having the environment in which to raise children).
6. Culture - it's not cool to have children. Now it's cool to travel the world and find yourself and experiment with as many partners as you can. This culture is not conducive to child rearing. Movies, books, music do not portrait happy families. Today, cool is to be 'different'.
7. Religion - christianity with its marriage and chastity laws is dying out. Is is being replaced by the secular world where monogamy and child rearing are not perceived as something desirable.
8. Lack of social support - as people travel to find jobs they do not live with their parents. Raising children without the support of grandparents is much more difficult.
9. Low wage growth due to women joining the work force. One man could maintain a family with children and his wife, 50-60 years ago. That is no longer possible unless you are in the top 0.1%.
10. Skewed dating market. Women are desirable by men until they reach 30-32. After that few men who want a family would try to know and marry a women because she is less likely to bear children and this is not a priority for her. Women are under much pressure to sort out their careers, know themselves and start of family by early thirties. The education and career 'system' is not optimised for mothers.
11. Global warming and reaching peak resources. We have reached the peak point in multiple natural resources and are destroying natural habitats at an alarming rate. This makes people less likely to see a hopeful future into which children have a place.
To summarise, we live in era of tremendous technological and cultural change where we reached the limits of our planet's resources in the way we currently use them and this has led to population shrinkage in the developed world.