PyCharm asks for python interpreter every time project is loaded

You can easily do it by going to File > Setting for New Projects > Project Interpreter:

There is no Default Settings any more, It's called Preferences for New Projects in the newer versions of PyCharm (my version is 2018.3.3).

For that, go to :

File > Preferences for New Projects > Project Interpreter

Setting the IDE default interpreter fixed the problem, as Vadim mentioned. Found instructions on how to do that here.

File | Default Settings | Project Interpreter.

This configuration sets the default interpreter for the new projects, note the Default Settings menu, it's not the same as File | Settings which sets options for the current project.

Not entirely sure why the default was unset, but at least now I can set it back again. The project default setting was being forgotten as well which is curious, but a problem for another day.