They never actually act like friends. Instead Anakin goes on about saving Obi-Wan and Obi-Wan denies it. They have the absolute worst on-screen chemistry of all time. ANH at least makes it seem like Vader and Obi-Wan really were good friends but the PT fails at actually showing it.
If you watch The Clone Wars animated series, you will see that they actually were good friends.
It doesn’t excuse how poorly the characters were handled in the film itself.
I shouldn’t have to wait years after the fact and then watch five seasons of a tv show just to understand the dynamic between characters that should have just been done better to begin with.