What were ASRIEL's motives again?

In the true pacifist ending, he wants to defeat Frisk so that he is the most determined being in the underground, and can reset the timeline. My question is, why? I might be misremembering something, or I might have forgotten something, but I can't remember his motives. Does he not like his cool-looking goat body? Does he feel guilty about absorbing everyone's SOULs, and wants to undo it? I can't remember.