Long ago, royals only associated with other royals. They held grand celebrations and attended exclusive political functions, consulting with other high-ranking officials regarding the countries they ruled. Children of royals didn't play with commoners' children and stayed relatively sheltered from the world outside their tall palace walls. Edward VIII believed it was improper for a princess to be educated with commoners, so the young Queen Elizabeth II grew up very differently from today's young royals, receiving her education privately at home.
Modern royals don't necessarily rule their countries, and they have schedules filled with a different list of responsibilities. They often work outside the palace and attend events and fundraisers with commoners. Traditional responsibilities and practices have been replaced with duties of public service, where royals regularly mingle with everyday people. In the past, Prince William, known for being a passionate humanitarian, has volunteered at a British Red Cross aid distribution center, and he spends his free time working with several charities. Some royals work in various businesses and industries, so it's entirely possible that a prince could meet and marry a commoner he works closely with at the office.
New generations of royals have openly expressed unfavorable feelings concerning old-fashioned traditions and values, and much of this can be greatly attributed to socializing with people outside the royal circle. British royals now attend concerts, restaurants, shops, bars and nightclubs like everyone else. They seem to be the first generation to be able to easily relate to commoners, widening their circle of potential paramours.