I don't replay the games often, mainly because 1. they're one-of-a-kind experiences, 2. I want to keep my Pokémon safe somewhere. Out of all my 3DS games, I have only replayed two - VC Crystal and Ultra Sun. Crystal I had never played before, but once I realized how good it was, I repeated the experience. I also admit I replayed that one to challenge a certain ninja at the Elite Four, because I got fully turned on to him in the short time between both playthroughs. Ultra Sun, I replayed to complete a Ghost mono-type run, which I had been wanting to do for a long time - and finally took the chance when PokéBank went free for a time after HOME's release.
Now that the Switch allows for multiple save files, I have also revisited Let's Go Pikachu for much the same reasons I replayed Crystal, ehuehuehue and now, after several failed attempts, I'm finally, FINALLY revisiting Shield. Since these are more recent, I can offer an amusing insight on how these runs went.
I didn't grow very attached to my Let's Go team aside from my Haunter and shiny Gyarados, but I had a Wailord of a time going through Kanto again with all the changes made to the region. I love how much freedom the game gives you as regards what events you can tackle, and how you can create your own story based off that! I just wish the variety of Pokémon in the game were greater. Conversely, I have grown VERY close to my new Shield team, but my motivation to go through the story is wavering now that I see it under a new light. I'm starting to hate how much the game railroads you and how obnoxious most of the characters (with the exception of Kabu, Raihan, Allister and Opal, and the rivals on a good day) come across on a second playthrough. My babies and I are spending more time in the Wild Area camping out, collecting berries and treasures and fighting Pokémon several times our size than actually doing the Gym Challenge simply because the latter implies going through that slog of a plot. Alas, I need to get them badges to get transferred Mons to obey me, so I'll try to enjoy the trip at least until Melony's Gym puzzle and the endgame events/post-game episode make me want to fling myself into the Sun.