In Purgatorio, Dante puts the source of the mythical rivers Lethe and Eunoe at the peak of the earthly mountain at the antipodes that is Purgatory. After purging yourself of your sins in ascending through Purgatory, you must wade through the two rivers before entering paradise.
The Lethe (Greek for “forgetfulness”) purges your memory of your sins. The Eunoe (“good mind”) enhances your memory of those good things you accomplished in life.
It’s a nice reminder of moving on in our own personal lives. You can’t really move on to bigger and better things until you put the old issues behind you. Heaven with constant reminders of the problems of the past is not really heaven.
There is also an interesting Buddhist flavor here, reminding you to live in the present. What’s done is done, you’ve accounted for it, now move on.
— Reader’s Note —
I encourage everyone to make the full journey from Inferno to Paradiso. Its best done, in my opinion, by choosing whether you want to read for poetry or read for late medieval Italian historical detail. Doing both at once can be overwhelming. Either way, if you do, definitely check out the Dartmouth Dante Project.