Your shoulder is the most mobile joint in the body, but also the most unstable. Especially after decades of wear and tear.
It's supported completely by thin ligaments and small muscles. And when these get tired or injured, your joint starts to "drift", which leads to clicking, grinding and aching.
Ultimately leading to an even worse injury, like a tear, that can take you out of the game entirely... If you don't find a way to support your shoulder properly.