Number 1 (for me) has to be the colors of Fall — they’re bright, warm, and make us (well, me, especially) want to be outside communing with Nature.
Number 2 might be the temperature — warm days, cool nights, always changing, something for everyone…
Number 3 is a little more abstract: the effect of short daylength on our brains makes us a little lazy and lethargic (or is it the warm sunshine?); long nights give us the perfect excuse to get more sack time. The end result is we might feel more rested and less stressed (or is it the warm, bright colors of fall that do that?).
Number 4 is the fact that Fall is harvest season — it’s a time of plenty, for both humans and animals.
Scads of berries, nuts, seeds, and fruits are available for primary consumers, along with a large supply of young, naive prey animals for the predators.
At other times of year, consumers most likely face food shortages, with scanty plant and animal productivity during winter and spring, and limited food supplies during much of the summer as plants and animals are growing the next generation.
I’m sure there are many more reasons why Fall is the best time of year — or do you disagree?