"This PC" always open in File Explorer.

Solution
Unless it reopens when you start PC for day, you appear to have answered your own question. All the folders in the row behave like that, you can close them all, then close file exploder and reopen and they all still closed.

If it does re open, try closing them all and restart PC and then see if they open again. The restart may change the settings for the folder, whereas a normal shut down/start may not save the changes.
What is your operating system in windows 10 open the file explorer click file click change folder and search options. in general click drop down menu of "open file explorer to". then you can choose between quick access and this pc
 
Thanks but what I meant was the file tree on the left is open when I open File Explorer. I've found that if I collapse the tree, close File Explorer and then reopen the tree remains collapsed, so it seems it responds to the state that you leave it in...

This_PC_file_tree.png
 
Unless it reopens when you start PC for day, you appear to have answered your own question. All the folders in the row behave like that, you can close them all, then close file exploder and reopen and they all still closed.

If it does re open, try closing them all and restart PC and then see if they open again. The restart may change the settings for the folder, whereas a normal shut down/start may not save the changes.
 
Solution