I guess people are just different I find running hard and free compared to picking things up and putting them back down again.
I get really bored at gym and just want to power through the exercises and get home and watch TV which unmotivates me.
The thing that gets me in running is you are running really well (e.g. 5 min) splits for 10km of 15km (your PB time) you can feel your body start to get tired but you have to keep pushing 5 mins per km for 5 more km's in order to achieve your goal. You mind is pushing your dead donkey body forward and your body is fighting to breath and move, at this point there is no boredom its absolute clarity and all that is pushing you is passion it really an indescribable feeling.
Although I do understand the gym your results aren't times they are 6 pack abs or huge biceps aswell as picking up a 200kg bar. For running all you get is sore knees and a skinny body for 10 times the effort.