One accepted theory is that they never did become domestic animals: They’re just wild animals willing to tolerate those who give them treats.

Another accepted theory is that they infiltrated human society with the building of granaries, which provided them with easy hunting while eliminating pests that humans needed disposed of.

My personal theory is that it was their curiosity that did it. Once they saw the Egyptians building pyramids, they couldn’t resist hanging around to see what the crazy humans would do next.