The whole premise of the game is pretty bad what with Isaac's mother wanting to kill her son. It doesn't help that Isaac is crying throughout the game.
On that note, every memory he has during level transitions are just horrible to look at. The poor kid just never got a break...
Ending 13. It depicts Isaac, his pre-insanity mother, and a skinny man who's presumably Isaac's dad, the first few pictures are of all of them living happily, then it gets dark, it shows Isaac crying by a chest, a demonic him in the mirror, and the very last photo? Isaac, with his mother setting her hand on his back to comfort him, looking out to the hill where an earlier picture showed the family picnicking at, that now houses a lonely grave, The End.
That's not a grave, that's Isaac's father returning home.
Or is it Isaac's father leaving?
The new endings of Rebirth also count. The first, number 15note If you're curious about the numbering, there are now 11 Womb endings instead of 10, so the previously-mentioned Ending 13 is now Ending 14, shows that Isaac has gone missing, with his Mother desperately searching for him. The second, 16, shows Isaac locked in his chest, as his breathing gets faster and harsher, and his heart beats quicker and quicker. Flashes are shown of him turning into a demon, and as his heartbeat slows down, Isaac morphs into a demon, and stays that way. A harsh Downer Ending.
Isaac can find food items that increase his maximum HP, all of which are rotten and at least two of which are dog food. The fact that they're referred to as "Breakfast, Lunch, Dinner, and Dessert" has some rather sad implications.
One item, a brass knuckle, causes Isaac's face to become badly bruised and occasionally replaces some of his tear shots with teeth. The item is named "Tough Love."
The Torn Photo item in Rebirth, which is a torn half of a photograph showing what appears to be Isaac's mother holding him. Picking it up increases Isaac's tear fire rate and makes him look absolutely miserable◊.
It's a torn family photo. Isaac's father has been torn off.
Some of the text flashes that randomly appear when you start the run, presumably written from Isaac's point of view, just ramp the sadness up with lines such as: "Mom?" "Where are you?" "Do you really want me to die?"