But then their population started decreasing earlier than Europe.

Could you elaborate on what you mean? Japan's fertility rate- right now- is actually slightly higher than Germany's, and Poland's, and slightly lower than Russia's. Granted, it's lower than France's and Britain's, but in those cases you'd probably want to compare against demographics instead of the total population.

Japan had 80 years of huge population growth instead of 50 years.

Actually its more like 40 or 50 years. Japan's growth rate started to decline in the mid / late 70's until deaths started to outpace births in 2005, 2006. Do you mean to say growth? Because the periods of exceptional population growth end in the 70's and early 80's and start a downward trend.

As for why, you're likely looking at a mixture of economic and cultural reasons, but this kind of question feels like it'd be better directed at a psychologist, sociologist, or an anthropologist. The short answer would be that Japan's economy had a meteoric rise in the second half of the 20th century that Europe couldn't match, coupled with societal expectations that are quite unlike what Europe has, and then ran into an economic crisis that they're still boxing with quite unlike anything Europe had to deal with.