By way of nature, the part of our brain that processes scents is also closely connected to the part that stores memories. That's why specific smells can almost immediately bring you back to a finite moment in time and make you feel nostalgic for, or triggered by, the memory associated with it. (For me its the smell of school bus exhausted on a chilly morning).
The spices that culminate in the now famous "Pumpkin Spice" include cinnamon, nutmeg, and ginger. The same spices that are commonly used in foods and desserts we enjoy during the fall and even during the holidays.
The way our brains work and the sheer scale of marketing we've been subjected to around Pumpkin Spice Everything is how we have such visceral reactions to the flavor when it hits the air each fall!