The short explanation on shingles is that when you get chickenpox, the virus that causes it,
varicella zoster, actually infects nerve cells. Your immune system fights off the infection, and you get over the pox, but the viruses remain in your nerves. When your immune system fades, (age, other autoimmune disorders, stress, other infections, chemo...), the virus can re-emerge from the nerves, causing the characteristic lesions along your chest, but it can be other places as well. Because it is erupting from your nerves, they are both damaged and stimulated, causing the intense pain and itching. Since the lesions are virus laden, you run the risk of transferring the virus to other parts of your body such as your eyes, where an infection can trigger blindness.
The virus is related to the virus that causes cold sores (which hangs out in facial nerves), and the same pattern of reemergence under stress, and herpes, ditto.
As to why shingles only occurs in 10-20% of the people who had chickenpox...who knows? Welcome to the wonderful world of individual immune systems, where everyone is different. Look at the current coronavirus; some people get it asymptotically, and clear it in two weeks, some don't clear it for months, some people have mild disease, some have life threatening disease, some die, some end up with permanent scaring in heart/lungs/intestines/kidneys, some folks end up with "long covid" that looks a lot like chronic fatigue, which is a permanent chronic condition.
Hope that this helps, and yes, please get vaccinated.
All the best,
Peter