I found my chemo so gruesome I would have liked to have rung a bell to have just got through it. In fact even getting through the the AC was a bloody relief.
I certainly get if you have metastatic disease this would feel very different.
The bell is not a celebration of being disease free nor never requiring another treatment. It's just a celebration of completing that particular treatment.
There is a little poem that goes with it. It's also on radiation therapy areas at some hospitals.
Ring this bell / Three times well, / It’s toll to clearly say / My treatment’s done, / This course is run, / And I’m on my way.”