You were hanging out with Kieran in the South-Eastern part of Kitakami. Kieran had gained a crush on you, but he was too chicken, or Torchic if you get the reference, to say anything. He had recently heard that in the Galar region, if you give someone an Applin, it means you like them. So he decided to hunt down, and I mean he searched the entire region, to find an Applin.

To his shock, and maybe luck, he managed to catch a shiny Applin, and a shiny Dipplin!

He was SO happy! He needed to give BOTH Pokemon to you. It would probably show you just how much he loved you.