The metaphor isn't being expanded. It's being actively changed to no longer apply to the original situation. No shared empathy is being built because the situation being empathized with is not the original situation that the spoons theory was originally propgatated by.

Building empathy is the skill of identifying with situations that are not one's personal situations. Arguably, "expanding" a metaphor to suit one's personal situation is the opposite of fostering empathy.

EDIT: Towards "Why should I care, every time I try to relate they say I'm not one of them?" -> The spoon theory was not explicitly to be related to. It was a way to abstract a very real problem that has been broadly minimized into a finite situation in order to explain a phenomenon that a healthy person explicitly does not need to deal with and cannot relate to. It would be similar to me explaining a technical problem with metaphor; I'm not asking for you to relate to the problem. I'm asking you to understand the problem.

As for why should you care? Because this person is in a lot of pain and is dealing with a lot of difficulties that I'm not dealing with, and in fact, upwards of 40% (more maybe) of people in my country (USA) have a chronic/incurable condition. The scale of the problem means that developing a compassionate, understanding view will greatly benefit a significant portion of people one may meet.