Now, at mid-career, I am beginning to understand the seduction of slowly fading to gray. Make no mistake, I am still very much involved (indeed, a little too involved), but I can hear the mermaids singing each to each. I would not say there is any bitterness yet, but I understand how such bitterness can arise. There is a kind relentless rhythm to teaching. You go over the same ground, plant the same seeds. And the students keep coming semester after semester. After the first ten years or so, you begin to realize that the entire enterprise has a momentum of its own that isn't particularly dependent upon your unique contributions.
Beyond that is the sense of futility that can accompany committee work. Hang around long enough and you find yourself revising policies you once revised until they resemble what you once felt compelled to revise. Sometimes, too, younger colleagues will propose something that once touched off the academic equivalent of World War Nine. Oddly, you notice that no one but you and few other survivors even seems to remember. Many years ago at my institution we had something that came to be known as the "Entitlement Wars" (don't ask). Few are those today who remember the blood that was shed, the ill will generated, the double-dealing. That does not mean the scars have gone away. It's just that hardly anyone remembers anymore how you came to have them.
Another source of bitterness is the awareness of how quickly you disappear. A few years after retirement and you may as well never have existed. After four years, the students have all changed, and after a half dozen your name will have become a few vaguely familiar syllables: "Professor Who? Didn't he have a moustache, or was it a beard?" Once, years ago, when I was working construction, a foreman asked me how long I thought I would be missed if I quit. To illustrate his point, he placed his hand in a nearby bucket and said, "You'll be missed as long as it takes the water to fill the space of my hand after I pull it out."
Ah, but this is all just mid-year gloominess. I need to get back in a classroom. I hate Winter Break.