Question

"what belief about marriage became increasingly common among middle- and upper-class europeans by the beginning of the 20th century?"

Answer 1

Before the begging and mid 20th century marriages were arranged. This particular practice started to decrease in the early 20th century due to the fact that young people were no longer "forced" to marry, but we're "introduced" to suitable partners and had the chance to see each other before the marriage took place.

By the first half of the 20th century, marriages were almost no longer forced or arranged, they depended on personal preferences and dates.

Nevertheless, nowadays there are still some countries, cultures and religions that still practice arranged marriages, but they're the exception not the norm.