Before Joker lost it he was an honest man trying to get some money for his wife & (unborn) child.
So he decided to join The Red- hood gang.
But on the night he was meant to start , he found out his wife and child had died.
But the gang didn't let him leave so he was forced to do it.

Inside an aced factory , he was made to fight Batman.
Then he tripped and fell into aced.

In " Suicide Squad" he stated that after electric shock therapy , he was left in a black hole of anger & memory-loss.

He forgot his wife but HATES feeling love.

That's why he doesn't love Harley. But still feels something...

( P.S. I got the info from Batman: killing the Joker / Suicide Squad.)