Societal brainwashing seems like a fair explanation.
However it can become quite annoying when you are married to a Japanese, and marriage (ours or her friends) becomes her favourite discussion topic.

It's interesting though to compare the societal brainwashing from country to country.
Japan also has food and money, which certainly come before wedding. Some topics are almost universally popular : sex, money, happiness, power, fame... But in others it kind be quite peculiar. Examples of national obsessions would be, I'd say, in "buying a house" for British people, "culture" for French people, "sports" for American and Australian people, or "caravan travel" for Dutch people.