Maybe his reasons for running are the same as Ann Trason's.
Trason, a 34 minute 10k high school runner(20 years ago!), was extremely talented. She may have been able to make an Olympic marathon team if she had put her efforts in that direction. Instead she chose to run ultras (I think she won the Western States 100 mile race ten straight years, challenging even the best male runners) Not knowing Ann or her reasons, I am only speculating: but, maybe she ran for the sheer enjoyment of trail running. Running through the woods like a child. Maybe Kawauchi enjoys the physical sensation of running sub 5 minute miles, mile after mile.
Everyone has their own reasons for doing what they do.