Do you remember when being human meant you were normal? Probably not. It's been three hundred years since the last human without a “demon helper” lived. When the barrier shattered, it's just how things ended up. It's not all that bad though, trust me. I mean, at least we aren’t slaves, right? That’s usually how the story goes. You may not have a choice in having a demon attachment, but at least you still have… well. Your freedom?
No human is ever born alone. Whenever a human is born, a demon is ‘born’ along with them. Demons attach themselves to humans — demons get a free vessel to use as a battery, and humans get to be a little less average. It's great. Sort of. For the most part. While there have been various stories of human hosts and demonic attachments that don’t exactly get along, it is generally regarded as a symbiotic relationship. The demon protects its human host, and the human host gives the demon a means to enhance itself or otherwise harness greater power reserves.
Humans can only host one demon at a time and demons are classified as Primal or Civil. Every demon has its own Dominions and can gain more during its residency within its human host. Human hosts have the ability to acquire Ascensions, which further affect both the human itself and its demon counterpart. Sound complicated? Don't worry. Most countries around the globe include “demon care” in their curriculum, assuming the demon itself wouldn't inform you of all the details during the course of your life. Demons are chatty buggers.
Successfully becoming pregnant often involves brokering deals with demons — particularly the one in your own body. After all, that demon will have to play nanny to another demon and share their space with someone else. Unwilling roommates? Yes. Some demons are more receptive, but most are like pissy cats that don't wanna share their host with another demonic entity.
Humans live longer lives. Partially this is due to demonic aid in medicine, but much of it is also due to the mysterious process of ascension.