It's hard to give a single answer that encompasses the motivations of all viewers across all genres of reality TV, but I do think there are a couple of interesting elements at play here that might be worth considering as you frame your answer.
With some shows, the appeal is certainly an opulent lifestyle. The Real Housewives, the Kardashians, Paris Hilton—these are people living lifestyles that most people can only dream of. The rich and famous have always been objects of fascination across cultures and continents, so this isn't a new obsession. Just think about the way people have talked about royalty and celebrity throughout history—Marie Antoinette, Lord Byron, Franz Liszt—the details of their lives have always been fodder for gossip among regular people.
Seeing people in these aspirational positions onscreen, especially in shows that often break the fourth wall, brings them closer than the curated picture viewers usually get—you don't just see the mansion, you see the room in the mansion that's under construction and has tarps everywhere. You see someone whose life is so different than yours lounging around at home, and realize you've got the same pajamas.
This particular element of viewership might also tap into why there are now so many popular reality shows focused on the lifestyles of the less rich and famous. Take Duck Dynasty, for example: It's not uncommon to see people wearing Duck Dynasty-branded merchandise who look like they might have walked right out of the show. There is an appeal in seeing yourself represented onscreen, especially when the cultural norm has historically been to exclusively feature people who are extremely rich, famous, and classically beautiful.
More broadly, people sometimes respond to these shows entirely because there is genuine catharsis in watching people have high-drama interactions. Even the most irritable regular person rarely flips a table in a restaurant, however much they might want to. Watching someone else so uninhibited might tap into something atavistic and primal.
In terms of global interest, North-American culture is especially primed for these shows to flourish for the reasons mentioned above, but also because "celebrity" is a disproportionately big element of our culture: we dominate most niches of the global entertainment industry, and entertainers from other countries often come here to try to "break America" when they feel they've gotten as successful as they can back home.
This isn't to say it's a strictly North-American phenomenon. There are many such shows originating outside the United States, and some of the most popular US shows are based on shows from other countries. One might even argue that game shows and unscripted documentaries—Antiques Roadshow‚ for example—are precursors to the modern era of reality TV, even if they're often culturally categorized elsewhere.