When all of the fixtures in a bathroom are the same age sometimes they all wear out at the same time. If a faucet begins to leak the shower may follow not long after.
This is not always the case, but it happens often. For example, last month I replaced a cartridge of a leaky Valley faucet in my neighbor's bathroom. This week he tells me that the shower (a Valley shower ) has started to leak.
Since the both fixtures were installed when the house was built we know they are the same age. So, the fact that they wore out at about the same time is no surprise.
Keep this in mind when you replace a part in a leaky faucet or shower. You may wish to do both at the same time and get it over with.