Here is one possible reason for being pleased to discover that one was unhappy in the past: Times of apparent unhappiness can lead to great personal growth. For instance, the hardest, most stressful time of my life was studying for my physics honors exams. However, now that the exams are over, I am glad to have both the knowledge I gained in studying, and the self knowledge that I am capable of pushing myself as hard as I did. (Would skills learned during the missing time be retained? Even if they weren't, the latter reason above would still apply). It would be devastating to lose the memory of any part of ones life, but I think there would be some satisfaction in learning that one had spent the missing time doing something difficult but worthwhile, even if one was not happy during that time.

You should be at least as pleased as you would be to discover that someone else was happy during that period.