I encountered a large whitetail buck not 8 feet off a paved bike trail in a suburban forest preserve. He continued to lie still and made no attempt to rise or run when encountering me and my 45 lb. dog at 10:30 a.m.. It's head was elevated but it remained lying on the ground beneath a large tree. What is the most likely reason for this behavior?

It could be a number of things. If the buck's head was up, his eyes clear, and he seemed alert (i.e., he was watching you), he was likely just resting. Other than vehicles or hunters, a healthy buck doesn't have much to worry about in Illinois. And in the forest preserves they see a lot of people. So, more than likely, you saw a healthy deer that was taking a break from feeding. Other possibilities are that it was injured or sick and did not want to move from its current spot.