Well for me, I kinda see the tickling of peoples feet as a sort of bonding activity if the character doesn't mind getting their feet tickled. Like it could be from childhood experience which they grow up in households and communities which bare feet, sandals, and flip flops are the social norm when it comes to foot ware in their local society. So they develop a foot fetish a early age and then start playing around with the feet of the people that mean the most to them and having their feet getting tickled back as well to show as a sign of social belonging and acceptance.