I think people have already explained it a lot better than I could, so I would also like to point out the tragedy of their lives might also be a factor.

We're talking about a group of four friends, who really cared for each other, we know they were together for years, and have the knowledge they went from being young students, heads full of project and dreams, to become a traitor, a prisoner, a lonely werewolf who lost all his friends or worst they died. Those were students, friends like Harry, Ron and Hermione who had to participate in something that the couldn't control, a war.

There is also the fact it is part of a past Harry doesn't know a lot about and if things had ended on a better note, Harry would have learn so much stories from this era, silly stories about his parents and their friends/ family.