SPOILERS AHEAD FOR THE LAST JEDI. BEWARE! At the end of Return of the Jedi, Luke Skywalker, Princess Leia, and Han Solo party with Ewoks as the Force ghosts of Anakin, Obi-Wan, and Yoda look at Luke in approval. The Death Star has been destroyed, Leia and Han are in love, and Luke is left a hero and one of the most powerful Jedi of all time. It's was a classic hero's ending and one that Star Wars viewers lived with for decades. If you'd just seen the first three movies and the prequels (and not read any canon books, for instance), you could imagine that Luke went on to train Padawans or have a family of his own; that after the battles he went through things would be more peaceful for him. Or, maybe you didn't imagine it at all, but somewhere inside you knew he was fine because of the ending Return of the Jedi gave us. At the very least, you probably didn't actively worry about Luke Skywalker for half of your life.
It's for this reason that Luke's death in The Last Jedi is so hard to stomach. Yes, it is the loss of a beloved character fans have known for years and years and that's tough enough on it's own. (Even with knowing that he'll probably come back as a Force ghost, the idea of Luke with that blue glow around him is heartbreaking.) And yes, Luke went in peace and died the way he wanted to, as Rey explains to us. But it's not just Luke's death that we have to mourn; it's the life he led that we didn't get to see — regardless of what he was doing, whether that was devoting himself to the Force or something else entirely. The life that for all these years we thought — or subconsciously assumed — he might have. There's also the parts of his life we did see in the film that weren't exactly pleasant that make the whole thing tougher.
Watching a movie, you see a hero triumph in the end and there's a feeling of finality. You went through a hard time with them and they made it, so there's a sense of relief all around. Watching the end of Return of the Jedi, most people probably don't think about what happens to Luke next in a literal sense, because the feeling that things are OK is enough. Watching movies — particularly movies with a hero achieving something that seemed impossible — has trained us to respond in this way.
That is, if the hero is meant to be human. If they represent the ordinary person put into the extraordinary circumstance. (With superheroes, while they can be normal to begin with there's a difference, as we're almost always lead to believe that their purpose is to fight and win and fight and win forever.) But when it comes to Luke, in the original trilogy he learned about Jedi and the rest of the galaxy along with the audience. Yes, he finds out he's a powerful Jedi from a very important lineage, but in the beginning he's just a farm boy. In the end, we know he'll keep being one of the most important Jedi ever, but the assumption is that he's already fought the worse battle of his life. (Harry Potter is similar. At the end of the seventh book, you think, "Whew! He made it!" Then Cursed Child comes out to continue the story, and, wow, he's still dealing with some pretty crazy stuff.)
But The Force Awakens and The Last Jedi take that feeling of finality and stomp all over it. Luke is not fine. We don't get to see much of his life play out, but what we do know is that he exiled himself to an island after his nephew, who he was training, turned to the dark side and that Luke very briefly considered attempting to kill him. Luke has seen some sh*t, so, if you think about it, it's not surprising that someone who has been through what he has would consider this. (Finding out your father is a powerful Sith Lord and engaging in multiple battles with him that were potentially going to end in death would do a number on your psyche.) But it is very dark and very different from the Luke we used to know.
It's also hard to see that he lived out so much of his life alone in exile. Luke triumphed! Shouldn't the achievements of his twenties set him up for life? No, they shouldn't. They rarely do for anyone. And while movies lead us to think that this is the case, real life absolutely does not. It's strange when you consider it's a movie that includes things like droids and Force projections, but Luke's story in The Last Jedi is oddly realistic to how real life plays out. Things don't become easy for you just because they once were hard. We all go through seasons in our lives. Everyone faces ups and downs even into old age. We all know these facts to be true, it's just hard to see them be true for Luke, too, because while he is human, he was still a hero. It humanizes him to a point that's difficult to handle.
And he's not the only one. Just look at Han and Leia. Return of the Jedi gives us hope that they end up happily together. And it sounds like they did for a while. They even had a kid. (Well, at least one kid, and he didn't turn out so great...) But audiences don't get to see the joyful portions of their lives, the ones that we believed deep down would happen. Instead, we see that they've separated and their kid has become a murderous member of the First Order. In the real world we (hopefully) can't relate to this exactly, but we can understand the idea of a couple that was together for years not staying together once they're older, or to the idea of parents going through estrangements with their children. Real world issues are mirrored in fantasy films all the time, but seeing them come back again for a second, even darker round is a lot to see your heroes go through.
Knowing that Luke died feeling peaceful after pulling off the most badass move of the entire franchise makes it easier to deal with his death. It's hard to accept that he's gone (or "gone" because, you know, the whole Force ghost thing), but knowing it's what he wanted helps. What's harder to accept is that his life (at least what we've seen before any potential Luke In Love: A Star Wars Story spin-offs) was such a bummer. If Luke had moved back to a farm and been chilling with his family or devoting his life to studying the Force and successfully training non-evil Padawans when Rey found him, it would have been easier to accept his decision to sacrifice himself. Instead, I'm left feeling like I missed out on his life. Like he's a grandparent I didn't ask enough questions about while they were around.
But, in the end, after moving through the stages, feeling the sadness, searching for answers and getting at least somewhat to a place of acceptance, something I'm left with is an odd feeling of guilt. If Luke went in peace, then who am I to project on to him what I wanted his life to be. He's the one who had to live it.
It's hard to not do that to those you look up to or care about in the real world. To want "the best" for them, to want them to make the decisions you wanted them to make. But that's not how it works, and The Last Jedi — amid all the battles and alien creatures and struggles between good and evil — shows us that. I didn't expect it to and I'm sad much of Luke's life was so crappy, but like the rest of us, he just did the best he could. That's not usually the goal of a hero, but it is the most human one.