Timbermare was created as a tribute to pro player Jamie Wakefield's wife Marilyn (nicknamed "Lovely Mare"), who died of ovarian cancer. Keep in mind that Magic sets are designed about a year in advance, and Planar Chaos came out about a year after her death, meaning that the card was designed almost immediately after the tragedy. The full story.
Being the creator of a multi-billion dollar game allows Richard Garfield to create some nifty cards. But there's more! The art of the first card was left undocumented by Garfield's request, which was for the longest time honored by the entire Internet (but can now be seen here◊). The second card speaks for itself, and if you don't get the art for the third, consider why the artist put four plates on the table.
When Elspeth has been cast out as a scapegoat for Xenagos' ascension and turned against by seemingly all of Theros, including Heliod, Ajani comes looking for her and becomes her companion on the quest to slay Xenagos, along with the leonin of Theros. Cemented by Solidarity of Heroes.
Even amidst the Downer Ending of Godsend, there's heartwarming to be found. Elspeth's breath slowed as he took her beyond the boundary of Nykthos, Shrine to Nyx and laid her upon the bare earth. He wouldn't let her die at the feet of the gods. Even in Elspeth's last moments, Ajani is still there for her.
Reviving Melody, which depicts about a woman rescuing her husband from the Underworld, telling him "Listen to my song, and you need never forge a golden mask to return to me."note In the plane of Theros, those trapped in the Underworld can escape by forging a golden mask, at the cost of the memories of their former lives.
Nahiri's defiance of Sorin's cynical take on their resistance against the Eldrazi
Any hope is better than none. Always.
Ojutai's So Proud of You moment as his student Narset surpasses him. Even if it could be argued he has ulterior motives of wanting to keep the truth of his subjugation of the former Jeskai and the existence of Ugin hidden when he implicitly exhorts Narset to planeswalk.
The Dromoka of Tarkir, in the new timeline, are basically a big, giant familial meritocracy in which dragons have a bunch of 'children' adopted, and everyone is welcome. Unlike most of the other Dragonlords, Dromoka has nothing but respect and protection for her subjects, and really, just look at the art for Dromoka's Gift. She's just so proud of the brave warrior to have earned one of her scales, like a parent at a child's graduation.
Another thing to consider in the Dromoka is that unlike any other clan, the dragons and non-dragons freely speak to each other, with mutual respect. Not even the Ojutai can claim that.
Zendikar Resurgent, from Oath of the Gatewatch. Finally, after all the horror of the Zendikar storyline, all the torment and suffering brought on by the Eldrazi, Zendikar is finally safe. Ulamog and Kozilek are gone. Zendikar can heal. The nightmare is finally over, and we have the Gatewatch on standby to take down Emrakul and anyone or anything else who dares threaten the safety of the Multiverse.
In a particularly odd version of this trope, some of Emrakul/Emeria's dialog in The Promised End implies that she gives Tamiyo the power to seal herself into the moon because some part of her is aware that the inhabitants of Innistrad don't want to be remade in her image.
Emeria: "This is all wrong. I am incomplete, unfulfilled, inchoate. There should be blossoms, not barren resentment. The soil was not receptive. It is not my time. Not yet."