As we do this inner work of casting out the false beliefs and agreements in our mind we become happier. And as we become happier much of the motivation we had to do this work slips away.
With less of the motivation, we do less of the work that got us there. This makes us easier prey to fall back into old patterns. Why does this happen is the question to ask.
It is because in the beginning most of our motivation originated from pain. As we progress, we have less pain so we do not have the painful suffering to motivate us. Then how can we maintain our practice and stay on the pathway to greater happiness?
As we evolve, then so our motivation will have to evolve from pain to pleasure and love.
New home play assignment. Go do something fun, that you enjoy, that is pleasurable to you.
Make love with your partner if that fits the assignment. Create Fun, Laugh, be Silly, combine them.
Then build the desire to feel more of that.
Practice often.