we tend to feel that the notion of a "multiverse" is a sensible idea, though we couldn't possibly defend that intuition on theoretical grounds. we're not nearly that good at math and physics, though we would like to be. but it's rather easy to imagine, at least, separate Universes that differ from each other with respect to their physical laws. indeed it's possible to imagine two Universes both governed by the so-called Standard Model—hence, two Universes with the same overall "laws of physics"—but which are nevertheless quite different from one another because of small differences in the physical constants that govern the complex interrelationships between physical quantities. it is thought, for example, that slight changes to the fine-structure constant (https://en.wikipedia.org/wiki/Fine-structure_constant), which determines the strength of the electromagnetic force between particles, would result in Universes where atoms simply cannot exist; subtler changes in physical constants could produce Universes with radically different periodic tables. any slight alteration in the balance of competing forces among physical particles, from one Universe to another, could mean a radical difference in the disposition of matter.
now imagine trying to tunnel from one Universe into another. could that even work? for the organic beings of Earth, it's practically impossible to imagine—human bodies are a delicate arrangement of chemical reactions that can be knocked into permanent disequilibrium (i.e. DEATH) by relatively trivial upsets, like a bit too much carbon monoxide in the atmosphere. how the heck could you somehow manage to travel to a Universe where matter itself is different? even a trivial change could be fatal, at least given enough time—imagine, for example, being transported into a Universe where technetium was stable, but molybdenum wasn't. you'd get zero dietary molybdenum, and after a while you'd develop an unusual deficiency syndrome, practically unknown to Earth because trace molybdenum is everywhere to be found here.
any process of transition between one Universe and the other, therefore, must involve a singularity—some kind of break or discontinuity in the continuum of change between one Universe's physical laws and the other Universe's. in my hypothetical example, there's an unavoidable discontinuity between the Universe where molybdenum is stable and the Universe where technetium is stable; no smooth transition between these two sharply distinct states is possible. if it were somehow possible to pass through this singular point from the Mo universe to the Tc universe, one can imagine coming out a different person on the other side—a person whose biochemistry did not require molybdenum, and who therefore could live without trouble in the Tc universe. but that person wouldn't be quite the same person, would they?
or would they? I suppose that depends on whether you believe there's an intangible soul or not, for the soul could perhaps maintain continuity, not being subject to the unavoidable discontinuity in physical existence.
I write this partly in a desperate attempt at rationalizing just why Kris is cyan-colored in the Dark Worlds of Deltarune. (you might simply be someone else, Kris—someone required to have a different biochemistry. maybe Dark-World-melanin happens to be blue?)
~Chara of Pnictogen