The assumption is that this will become routine, but the post-apocalyptic world is unpredictable. The brain can adapt to almost anything but only if that thing happens regularly. Almost every day, a survivor would have to make a choice that would have brought them to the point of tears and vomit in their pre-apocalyptic life. Right now, in our modern world of ultimate comfort and convenience, do you have a hard time deciding what to watch on Netflix? Then good luck making a decision when you're starving and one of your fellow survivors starts to look like a giant roasted turkey.
There are seven billion people on the planet. The virus/holocaust wipes out nearly all of them. A few hundred thousand stragglers are left on the planet and they're dropping off, too. We're not built to be alone, and as appealing as the thought of every annoying douchebag you hated in the Before Times being wiped off the face of the earth is, being this isolated would suck. Humans have built an unspeakably complex society, with every aspect of our lives directly linked to the efforts of others.
How much of your day is made tolerable by the efforts of other people? Coffee brewers, podcasters, plumbers, and pot dealers -- All. Gone. Humans feed off each other, and when the stream of information is cut, the "human" in us starts to fade out. Will Smith's character in I Am Legend would be delusional and have no impulse control. You kind of see it in his tableau of mannequins in the video store that he's assembled to achieve some semblance of normalcy. In reality, his madness would be much more like that of a man in long-term solitary confinement, but the cell is the entire city of New York. Loneliness would soon start to chip away at your sanity.
Would have lost his mind and eaten his dog within a month.
The Lone Wanderer type is a favorite of the post-apocalypse genre. It's what the Fallout series is all about. Max Rockatansky of the Mad Max series has a better relationship with his car than he does with other people. The sole survivor trudges through the [insert your preferred wasteland], finding it in them to somehow trudge on. But what post-apocalyptic fiction is saying about Lone Wanderers is that they basically have a superpower. They've somehow been able to remain relatively sane without humanity's regular social structures. No amount of survival training can prepare you for a potential lifetime of loneliness and sorrow. Tom Hanks in Cast Away is so desperate for anything resembling human companionship that he inadvertently builds a friend out of a volleyball and his own blood, and gets into an argument with it. It feels so human to argue with someone again.
20th Century Fox
"What you looking at, bro? You wanna fight, bro?"