I found this post on r/pokemonanime that is under the title “I don’t understand why Serena kissed Ash”
With 64 upvotes, this is this user’s explanation that will be bound to make the anti Amour squad happy:
“I actually think it was meant to close out Serena’s arc and doesn’t really lend itself to Ash x Serena shipping as much as people think it does.
Throughout XY Serena not only had issues telling Ash how she felt, but she had issues being honest with herself and people around her in the first place. Serena kissing Ash was her taking control of a situation. She did something she wanted to do without second guessing and over explaining it and just did it. And she was able to do it because Ash gave her confidence throughout their journey.
I don’t necessarily see it as romantic, Ash’s only love is Pokémon, no matter how bad people want it to be romantic it wasn’t. Because the writers would never put Ash in a fully romantic situation. Which is why I think all theories on how the anime ending with Serena x Ash being endgame are silly, Ash is never going to be shown with a girlfriend or anything of the sort. You can fantasise and theorise but it’ll never come to fruition.
But anyway, I got off track. Serena’s kiss wasn’t meant to be “hey I’m in love with you” it was closing out her arc, her saying thank you for an incredible journey and experiences and for his friendship.
This is further proof in her Journeys return - they only met briefly but their interaction was clean and fresh. She was simply happy to tell him what she was up to - no stuttering, no jealousy, just happy to tell him she’s still working to achieve her goal.
Her talk with Chloe and hearing her development was also confirmation that her journey with Ash was about her own self confidence. The reason the crush/kiss is never referred to is because it doesn’t need to be. If you watch it without shipper goggles you can see what it was for.
This isn’t meant to rattle anyone. I know this sub has a lot of hardcore shippers, but Ash is not that type of character and Pokémon isn’t that type of show, so I don’t buy into it, nor support the theories. Take this how you will, but this is how I see it.”
This user claims that Serena kissing Ash was her “letting go” of her crush. Does it make sense? No, not from a shipping context. But Serena kissing Ash as a way “to kiss her childish love goodbye” kind of makes sense, which further explains her lack of blushing and interest in Ash in JN105.