German Shepherd Dogs are a highly active and intelligent breed that requires proper care and attention to thrive. Leaving a German Shepherd Dog outside for extended periods of time, particularly during the day, can have adverse effects on its health and well-being. Here are some of the reasons why it is not recommended to leave a German Shepherd Dog outside during the day:

1. Physical and mental health: German Shepherds are social animals and require regular human interaction to remain happy and healthy. When left outside for extended periods of time, they can become bored, lonely, and even develop behavioral problems, such as destructive chewing or barking. Additionally, being exposed to extreme weather conditions for long periods can lead to physical health problems, such as heatstroke or hypothermia.
2. Lack of exercise: German Shepherds are a high-energy breed that requires regular physical exercise to maintain their physical and mental health. When left outside for extended periods of time, they may not have the opportunity to engage in physical activity, leading to behavioral and health problems.
3. Inadequate protection: German Shepherds are a highly protective breed and are often used as guard dogs. When left outside for extended periods of time, they may be vulnerable to attack from other animals, including wild animals, and may not have the opportunity to alert their owners to potential dangers.
4. Potentially dangerous situations: German Shepherds are known to be escape artists, and when left outside for extended periods of time, they may become restless and try to escape their yard or kennel. This can lead to potentially dangerous situations, such as getting lost or being hit by a car.

In conclusion, it is not recommended to leave a German Shepherd Dog outside during the day. Instead, provide them with adequate exercise, socialization, and attention to help them thrive and live a happy, healthy life. If you need to leave your German Shepherd outside for brief periods of time, make sure they have a secure, sheltered area to protect them from extreme weather conditions and provide them with plenty of water and toys to keep them entertained.