Here is that paper I promised. It's right around 900 words.
After the end of a long battle between PokeShipping and PalletShipping, and the Original Series came to a close, Ash left Misty, Gary and everyone else to go to Hoenn, bringing no one with him but Pikachu. Once Ash finally set foot in Littleroot Town and met May, the Advanced Generation truly began.
When Ash arrived in Hoenn in AG001-Get the Show on the Road, he first went to Professor Birch's lab to have his Pikachu healed. This is where he met May, who would receive a Torchic as her starter, and travel with Ash for the entire way through Hoenn. May was very open to the idea of her and Ash traveling together alone, and talked to him constantly for the first few episodes. Later, May's brother Max joined them, much to their initial disapproval. The days went on, and Ash was reunited with Brock, whom he travelled with throughout the OS and will travel with for a very long time to come.
After showing disinterest in Pokemon battling and having only embarking on a journey to travel, May finds herself entering contests and competing with rivals Harley, who hates her with a burning passion, and Drew, who has given her a rose each time they meet. This started the theory of ContestShipping, which is considered one-sided canon on Drew's side. Many of May's hints towards the ship could easily just be surprise, thinking, or accepting the current situation.
May gets progressively better in contests with Ash's guiding hand. The two meet a couple with a knack for identifying future couples, Oscar and Andi, in AG072-The Bicker the Better. This is supported by the fact that the two identified Jessie and James as a future couple, whom have been married in the canon Electric Tale of Pikachu manga. The couple are the first canon AdvanceShippers, believing Ash and May to be a couple. The two react to this by denying it and arguing throughout the rest of the episode, and bashfully make up at the end.
Time passes by, and in AG114-Date Expectations, the group is out on an island while traveling to the Grand Festival in an episode revolving around love. During the episode, the group is separated, with Ash and May being alone. During this, May starts to pretend to do a nature show. This shows us that she is comfortable with Ash and is ok seeing her act like this. She also blushes at the thought that they might be stuck there alone, possibly liking the thought.
Time goes on, the Hoenn league and the Grand Festival are over, and in AG133-The Right Place and the Right Mime, the group separates, then rejoins at Ash's house in Kanto.
At the end of the Advanced Generation, Ash and May tie a contest in AG192-Home is Where the Start is. The two end up splitting the ribbon in half and keep it to remember each other by.
M09-Pokemon Ranger and the Temple of the Sea was pretty much the closest Pokemon has ever gotten (and probably ever will get) to two main characters loving each other, save for Pokemon Live, which isn't considered canon and most PokeShippers don't even bring it up. Every major Ash shipping has a song. PokeShipping has Misty's Song, PearlShipping has High Touch and NegaiShipping has Rivial Destinies. AS didn't really get a song. It got a whole movie, in which Ash and May swam with Luvdisc, which is explicitly stated in the games as being a sign that two people love each other. Not only that, but Ash is recognized as having the closest bond with May, something amazing because her own brother, whom it is clearly stated multiple times that they love each other very much, is right there with them, meaning Ash and May have a closer bond than Max and May do.
All of this leads up to what would be called a bittersweet finish at best and a real kick in the nuts at worst, as we are reminded in Together We Make a Promise that this was the last of the Advanced Generation and Ash and May would be leaving each other. Ash leaves for Sinnoh with Brock, Pikachu and Aipom, May leaves for Johto with Drew, Harley and her team. The two will not meet again except for 3 episodes in Diamond & Pearl. Together We Make a Promise tells us not to lose hope, and we are treated with a scene in DP076-A Full Course Tag Battle that shows us how much Ash and May meant to each other, as the two kept each of their respective halves of the Terracotta ribbon they won together, and both tell of how many times just looking at it has given them the confidence boost they need to win.
During the time they spent in Sinnoh together in DP076, May gave Ash a wooden Teddursa as a gift. It is clear that Ash cherishes this gift, as we see at the beginning of BW001-In the Shadow of Zekrom that he gives it the honor of sitting on the same shelf as the trophy he got for winning the Orange Islands league, the highest award he has ever received. No other items clearly identifiable sit on that shelf.
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