i have two headcanons about why sans’ HP (hope) is so low
sans was a happy kid with average HP, enthusiastic about science and fascinated by the surface. when he was old enough he became Gaster’s assistant, along with Alphys. they worked on ways to enhance underground living and researched humans, the surface and timelines. It was only after Gaster’s death (however that happened) that he started losing hope. He invents a machine to bring Gaster back, but fails. Over the years his HP slowly dwindles. He quits working as a scientist, his knowledge about timelines affecting him mentally and any faith in a future above ground is reduced to nothing. With almost all hope lost, he adapts a nihilist attitude and treats life as a joke. Papyrus is the only thing that keeps his hope from lowering any further.
sans seems to often get in the way of Flowey’s fun:
“His garbage brother. You know, Smiley Trashbag. DON’T. Let his brother find out ANYTHING about you. He’ll…well… Let’s just say he’s caused me more than my fair share of resets. Stay away from that guy.”
Flowey is sick of sans’ shit and finds a way to reset to an earlier time. He attempts to eliminate sans from the game permanently by sapping his BASE HP as a child, when sans is most vulnerable having no knowledge of him. Flowey confronts sans (8) and pap (baby) in Snowdin Forest where they had been living since appearing out of nowhere. sans was wary of other monsters and feared Snowdin. sans flees holding tight to pap, but Flowey ambushes the two.
after being thrown around a bit, sans finds his magic out of rage and protection over papyrus. he gives flowey a bit of a beating until he’s nice and pissed. sans gets hoisted up, pushed into a tree. bound in vines, sans is held still as vines stab at his chest and begin destroying his soul. his base HP drops fast. on the edge of death sans has flashforwards (the game causes him to foresee himself in the future, papyrus older, flowey toying with him, other events from the various plots) the memories-to-be fade with every hit point lost - as if they’re being deleted. a glimpse of his little bro crying in the snow delays his rapid falling hope just enough for his arm to break free of Flowey’s hold and send a field of bones up out of the snow and through Flowey. Flowey’s health is too low and forces him to give up, he leaves the timeline. sans is left with 1 hp, but he and pap are alive. they head towards Snowdin. this theory has a few holes in it but im okay with that.
The reason why his AT and DF is also at 1 is because he cant be bothered to train like Undyne or Papyrus (???)