Because I've been floating on this ship since 2002. Anakins hair is all tangled/dirty/gross after a mission Maybe Anakin gets injured or he's sick and can't really focus to take care of himself so Obi-wan does it for him or Anakin is having a bad day and Obi-wan just wants help him relax. Doesn't matter the reason.

Doesn't have to be an established relationship. It can be pre or just gen but I would like to see cuddling after Obi-wan washes his hair to show Anakin that he cares whatever the writer wants their relationship to be.