What many of you don't realise is that uncooked rice is a natural home to millions of tiny organisms known as Oryza mustelida, or the ricebadger. Ricebadgers are about a tenth of the size of a normal badger and thus smaller than a human eye, so you can't see them mixed in with the grains of rice, but they're there all the same.
Most of the time they are harmless, just naively going about their badger chores without a greater understanding of the ties that bind them. This all changes once you boil the rice. On doing this, the ricebadgers understandably get quite upset, as would you if your city and everything in it was cooked for ten minutes in a pan of water at 100 degrees celsius.
When everything has cooled back down these little guys are pretty much in a state of shock, but they're not willing to take such a beating again so they start getting organised, forming the first basis of unions in their underground jazz bars. From there the contingency planning begins and although it takes a while to build Andersen shelters and form successfully self-organising collectives, by the next day they've got some good scaled down solidarity going on. The trouble for them is that now the immediate danger has passed, the ricebadger population aren't ready for an uprising. The fat are getting fat again already and although they remember how you recently tried to stamp out their race, Diagnosis Murder is on ITV and for now that wins out.
Of course you have no idea about any of this unless you look at your rice through a telescope, and who does that? Unwittingly, you reheat the rice the next day. Ricebadger Command was deployed for this very moment though, and as you microwave their oppressed people, they're ready. Those who can, rise up. You eat your rice but little do you know, that on this darkest of days for badgerkind, you're also chowing down on their special operatives, civilian militias and anyone fit enough to fight.
Soon ricebadger commandos have infiltrated all your important parts like the spleen and the large asparagus, and they chew through your guts like any angry badger would after you murdered his family in cold blood, laughing, right in the middle of Quincy. They're only small so there's limited damage that their last stand can inflict, but it's enough for a night of you vomiting up their brave little bodies with the rest of their ill fated civilisation, god rest their souls.
Anyway that's why you shouldn't reheat rice. HTH.