In my headcannon, they hug him to warm himm . Their Cutie Marks good as their magic from their hearts warm themselves, the Pokemon, and Ash. And Ash earned their friendship through bravery but most importantly compassion.

In my headcannon, Ash befriends Rainbow first. Fitting since they’re sassy yet sweet. And Flurtershy be second. Fits they be pals due to being animal carers.
Pinkie. Zero shock. Jolliness.
Ash with Applejack. Zeor shock> Spunk.
Rarity. It be grand if Ash try to calm Rarity when she overreact. And act as her conscience whenever she hurt Sweetiebelle.
Twylight. It rock if Ash help Twylight redeem herself for her sins. He’s capable of mercy such as forigivng Mewtwo even after his past errors, and also to Molyl evne after her errors.

Awww. Sweet. A heartwarming scene I wish came to be.