Volume 3: Kiba's friends in the research facility all encouraged him to move on and live in the world for them.
Volume 6: Asia declares her love for Issei, while giving him her first kiss.
From the same volume, after Asia is saved, Xenovia glomps her and begs Asia to never leave her alone again. To put this in perspective, three volumes ago, Xenovia was calling Asia a witch and a heretic, and threatening to kill her.
Issei asks the Archangel Michael himself (who now rules Heaven) if he can allow Asia and Xenovia (now Demons and therefore racial-enemies to Angels) to pray without pain, knowing how much they love God; a request that Michael graciously grants.
"The Almighty Himself did believe in forgiveness and redemption; so I suppose exceptions to this outdated punishment is acceptable."
Volume 7: Akeno finally was able to accept and forgive her father for returning too late to save her and her mother in their time of need.
I find it hard to believe that no one bothered to write the part where Issei refuses to take advantage of Akeno and have sex with her despite her wanting him to, and all because he realized she was just doing it to forget the issue with her father. This practically multiplies her love for Issei and helps her get over her emotional baggage as well as fix, or at least begin to fix, her relationship with her dad.
Issei, Vali, and Odin in one room with Odin mentioning that he's willing to bet on the new generation. Seeing as he was introduced as A God Am I who laughs at the younger generation, this says something about him. Seeing an old man watching the younger generation grow up sure is heartwarming.
Volume 10: All the other girls comfort Issei to get over his past trauma with fallen angel Raynare, and encourage him to confess to Rias.
Also in Volume 10, Sairaorg's mom finally woke up after being in a coma for a long time.
Volume 11: When Kuroka is messing with Issei and Koneko, Ravel storms into the room. You would expect her to throw around some tsundere lines about Issei, but that isn't why she's here. Ravel walks up to Kuroka and threatens her to her face to make her stop teasing Koneko, or else. Kuroka backs down.
Volume 12 has Ophis finally happy to be with Issei after an entire lifetime of having no friends whatsoever.
Kiba gets to watch the television and it shows that the children are waiting for Issei to rescue them and believes in him. This makes Kiba break down into tears for Issei.
Issei may have died, but his will is still inside Rias' pawn chess pieces to the point one of said pieces becomes Ascalon and encourages Kiba to fight on. Then later on, everyone present had a piece of the pawn with Issei telling them to fight on as well.
Issei comforting the kids in their dreams that he will come and rescue them by drawing a circle in the air, point in the middle and sing the Oppai Dragon song. At first when he wakes up again, he thought that it was all just a dream until he hears the kids singing the Oppai Dragon song. Normally, he would be just displeased but this was the one time when he actually couldn't hold his tears back because he was able to bring smiles to kids even though the whole Underworld was attacked by the Khaos Brigade.
And the one that made this all possible was Great Red who found both Issei and Ophis at the Dimension Gap.
The reason why Tannin decided to become a dragon-devil: preserve the dying race of a certain dragon species.
Volume 14: The prologue focuses on the fact that all of the girls have gotten much more attached to Ise after they thought he died. And it isn't in a sexual way, they simply want to to be closer to him as much as possible, like they're trying to reassure themselves that he's still there.
Also from Volume 14, Ravel's talk with Issei about how she knew he would rescue her, just like how he came to rescue Rias in Volume 2.
Volume 16: It doesn't matter for the group whether Gasper's Nightmare Fuel self is scary as hell, it's still the same Gasper who they knew and that won't ever change for them.
Granted it was hilarious on how it came to be, but both Ddraig and Albion finally reconcile after engaging in their own Forever War with each other.