I'm going to the dentist tomorrow. It's such a pleasure.
Since when did it become a pleasure for you?
Since when has it become a pleasure for you?
The present perfect version could be better as we are talking about current situation.
The preposition since in the examples further indicates that the change or adaptation could still be on-going.
If we want to use past tense, in a different situation, it could be
[W]hen did it become a pleasure for you?
The subtle difference between present perfect and past simple is explained below.
Present Perfect Simple
Unfinished actions that started in the past and continue to the present
Past Simple
Finished actions
https://www.perfect-english-grammar.com/present-perfect-or-past-simple.html