Sometimes I want-to-want to write. So I force myself to sit infront of my editor and write. But I cannot. A large part of it could be just laziness, but I think there is more to it.
Usually, the things that I want to write about are philosophical, but ocassionally they will involve a personal experiential component. Something that has emotions attached to it.
The act of writing has different effects on different people. For some, it can strengthen their connection with the contents of their writing. For others, it can make them feel like they are losing the feeling attached to the contents of their writing. Sometimes you want to hold on to these feelings, even if they are difficult.
The heaviness and sadness that these ideas bring can be a source of strength - The strength to say "such is life" and continue.
When you offload this feeling, you can end up unconsciously resetting your expectations. At least that is what I think. You can end up feeling like you have to start over. Suffer through it again.
I feel most of the time what you want is the memory of having suffered so that you can reminisce about it, and feel better that you had the ability to suffer and come out of it. But not go through it again.
Expression of any form, not just writing, can sometimes take that feeling away.