Running on a treadmill feels easier physically because the ground is being pulled underneath your feet and there is no wind resistance or natural incline (unless incline is increased manually). Running outdoors demands more from your body because you are propelling your body forward stride for stride. Many treadmills are padded, making them a good option if you are overweight, injury prone, or have knee or joint pain and want to decrease the impact. You can better simulate outdoor running by setting your treadmill at 1% incline. I feel that is mentally tough to run on the treadmill due to the monotony and it is easier to distract yourself when running outside with the scenery and new terrain.
Top 5 Reasons To Run Outside
1. Enjoy scenery/weather
2. Time Flies
3. Better Prepare Body For A Race
4. More Challenging/Increases Motivation
5. Expends More Calories Than The Treadmill