I'm sure some of the causes vary by country, but for the case of Spain (with one of the lowest fecundity rates in the world and one of the widest gaps in Europe between the desired and the final number of children per person), the reasons pushing down the numbers (according to several studies conducted in the past decade) are:

• Lack of financial stability. Having children is too expensive and state support policies are too small.

• Lack of job stability. Even for people who work, a lot of the time it's in precarious conditions and without long-term prospects. Labour rights were severely eroded in the period after the financial crisis of 2008.

• Difficulty to rent or buy a home (much less one to start a family comfortably). Prices (especially in the cities where people can realistically find jobs) are unrealistic. People are often forced to live with parents or in small apartments with roommates until well past 30 years old.

• Very poor home and work life conciliation. As mentioned earlier, labour rights are not in a very good place right now, so even when the financial situation can allow it, finding the time for being a parent can be unaffordable on itself.

• Difficulty to find a stable partner with whom to form a family. At an individual level, education, work and other factors delay the average age where people start making long-term life plans. This affects the nature of relationships, too.

Also relevant that the amount of people who just don't want kids has remained stable around 5%, which is not very significant compared to other factors.