This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Old Man's War
The Old Farts share a wide variety of backgrounds and interests. Alan is an astronomer (who learned early in life that there was no money to be made studying the stars), and has all of the constellations memorized. As they are about to leave the Solar System, never to return, he has to come to terms with the fact that the stars will never look the same to him again.
Alan: And you lose other things, too. I mean, in about a minute, all that time I spent memorizing the constellations will be wasted effort. Thereís no Orion or Ursa Minor or Cassiopeia where weíre going. This might sound stupid, but itís entirely possible Iíll miss the constellations more than I miss the money. You can always make more money. But weíre not coming back here. Itís the last time Iíll see these old friends.
A CU attempt to retake Coral ends in unmitigated disaster, with only a single shuttle making it to the planet, and even that one is shot down and crashes violently. The only survivors are John and Alan, and neither of them are long for the world given their injuries. Alan, realizing their situation, has one last thought to share:
The operation to stop the Eneshan invasion in 'The Ghost Brigades' is a combination of this, Nightmare Fuel, and a Moral Event Horizon for the people who planned it. It is completely unsurprising that Jared had a Heroic BSOD halfway through.
Stage one was abducting a child. Stage two was sterilizing the child. Stage three... well, the first two stages were happy fun time in bunny land by comparison with stage three.
Jane Sagan visits Charles Boutin's family grave with Zoe after Dirac and Boutin have died.
The eulogy for Enzo Gugino in 'Zoe's Tale'. Made even worse by the fact that the last poem he sent Zoe could well be interpreted as a marriage proposal.
Made even more poignant when you go back to 'Last Colony and remember how little Perry thought of that character.
In The Human Division, Wilson is having drinks with Danielle Lowen, when he lets slip that he was friends with John Perry. He explains that they joined at the same time, along with several others, and formed a clique called "The Old Farts", and mentions that of the original seven, only three were still alive.
Wilson: "They told us when we joined that three quarters of us would be dead in ten years... So strictly on a percentage basis, the Old Farts beat the odds." He drank.
The short story "Hafte Sorvalh Eats a Churro and Speaks to the Youth of Today" is, as the name might suggest, mostly silly, with an alien diplomat taking a break during her lunch to chat with some human schoolchildren on a field trip in Washington, D.C. One young girl, Hannah, asks her if she is a monster, explaining that monsters fight and wreck things. Later, Hannah asks if there will be a war, because her father thinks there will be a war, with the aliens trying to wipe out humanity.