All three of them have heartbreaking pasts.
Meowth’s was listed above.
James had abusive (at the very least neglectful, but they didn’t care how he was treated so long as he awarded them prestige) parents who had engaged him to a sociopath, who wanted to whip him and change everything about him to be more “presentable.” He ran away and was on the streets for a long time before he finally joined a crime ring.
Jesse was raised by a single mother, and the two of them were so poor that they rarely had actual food; her mother would make her a “feast” out of snow in the winter that Jesse considered to be a treat because that’s how badly they were starving. If the audio dramas are to be believed, Jesse’s mother was also a member of Team Rocket, who disappeared (read: died) on an expedition searching for Mew, leaving Jesse alone. And then Jesse, like James (and Meowth) was so desperate for a means to survive that she (inadvertently?) followed in her mother’s footsteps and joined Team Rocket/a life of crime just to get by.
“Zany villains” they may be, but Jesse, James, and Meowth are the three deepest characters on the show. I love them.