Here's a breakdown of the factors:
* Early Purpose: Originally bred for bull-baiting, where dogs would fight bulls, a strong, powerful head was desired. Breeders favored dogs with wider heads and shorter muzzles for increased bite force and stability.
* Aesthetic Preference: Later, the breed was developed for companionship and show, with breeders valuing the unique and unusual appearance of the head. This led to further emphasis on the exaggerated features.
* Genetics: The egg-shaped head is largely due to the shortening of the nasal bone and the increased width of the skull. This is achieved through specific genes passed down through generations of selective breeding.
* Health Considerations: While the head shape contributes to the breed's unique look, it can also lead to health issues. These include dental problems, respiratory issues, and eye problems, due to the narrow muzzle and compressed facial structures.
It's important to remember that while the "egg-shaped" head is part of the breed standard, responsible breeders are working to minimize the health risks associated with the breed's extreme features.