First, remember that this is an academic study so several psychological theories are thrown around throughout the article. I’m going to consider just one, that I think actually helps us look at this finding in a new way.
To me, this means it’s all about expectations. There’s actually some research to back this up. Psychologists studying the relationship between individuals’ life satisfaction and major life changes have found that people often experience an increase in satisfaction in the year prior to the birth of a child, but this often drops within a year of the birth. However, this doesn’t necessarily mean they end up less happy than they were when they were childless, it just means that they experience a decline in happiness once the reality of parenthood sets in.
For more interesting information about the focusing illusion and other psychological thoughts, check out this great book by Daniel Gilbert: