Question: Why is it written in the sources that the work itself for the Creator or for His sake is considered a reward?
Answer: Because a person reaches his highest spiritual degree at the same time. He becomes eternal and perfect like the Creator, is honored with connection with the Creator, learns what the upper force is and in general what the entire universe is, the entire upper world. He rises to the degree where he is not limited by anything.
This can be compared to the work for some great person where working for him becomes a pleasure.
From KabTV’s “Spiritual States” 5/31/22