TLDR: Fertility rate dropping is bad. We should have good children and families because its most likely your calling and will help improve your society.
People in this thread have already mentioned the materialistic and economic downsides to fertility dropping. Less people means less workers, less geniuses, less people to help solve problems.
There's more to this than the mere material reality. I believe we have a moral obligation to contribute to our family and to our civilization. Having and raising good children is a large part of how we do that. Why? Its good for you spiritually. If it wasn't for the contributions of ancestors and to past and present individuals in your society, you wouldn't exist. Much less have the quality of life you have today. Being a societal leech is bad for you.
"The world/my life sucks! Why would I bring someone into the world if they're just going to suffer?"
  1. Suffering isn't inherently evil. It can help build character and strengthen you. Some of the happiest, most virtuous people on earth are living in poverty and struggling every day. For the people who aren't suffering nearly as badly: This doesn't mean you should ignore those who suffer or go out of your way to cause it. Suffering can build your character and spirit too by helping to alleviate it.
  2. Building a family can help alleviate your mental and spiritual suffering. Many people struggle emotionally and psychologically because they aren't aligned with their purpose. Building a good, virtuous family is a part of the purpose of most people.
  3. Having as many well raised, virtuous children as you can manage will improve the state of the world. They'll follow your example and have good children of their own. Having all these new, good people will influence and propel society into a good direction. They won't create a utopia, but they could at least end the clown world we're living in.
Now does this mean that everyone MUST have children or else? Absolutely not. Some people simply don't have the calling to become parents. However, the calling of these people is focusing their efforts on a purpose that will help their societies and themselves to flourish. They should also avoid vices that degenerate themselves and the societies around them. These people are in the minority (I would say like 20% or less of the population because Pareto principle). Most people should have kids.
Does this mean you should have 12 children? No, not necessarily. If you do and you're able to raise them all well, God richly bless you! People should stick to however many they can responsibly handle (within a marriage of course). I think most people can do 3. Most people should do at least 2 to replace themselves and their spouse.