When you consider Ishamael's motivation I am minded of a story from greek mythology. I'm not to sure of the name of the guy but the greek gods cursed a man to have to roll a big stone/rock up a steep hill over and over and over again and every night he got the stone up the hill and every following morning it was back at the bottom of the hill.

Or for the modern minded think groundhog day without the happy ending.

If you were someone who does not get much joy out of love or companionship and don't really like people the wheel of time would really suck. You are born, you live your life and then you die, then you are reborn back into a life you don't really enjoy and then die then reborn back to the life you are starting to hate a bit more each time you are reborn. Eventually you would snap and want it all to end.

Ishamael does not seem to have access to his past memories but he is smart enough to figure out what endless rebirth would meen that he has been doing what he has been doing over and over again.

One of the reason's LTT/Rand probably does not turn to the Shadow at any turning of the wheel is that he actually gets some joy/satisfaction amid the sorrow of each life so he keeps comming back for more because there is good there among the bad moments and those good moments make the bad worth enduring.