The Meiji Restoration in 1868 gave the Japanese an incentive to go to Europe and study how to compete with European governments, but no wealth to travel there for leisure. In the 1920s, art nouveau gave Paris some charm for educated Japanese, but Japan had no passenger air service at that time; only diplomats, and increasingly, military commanders were allowed on Imperial Japanese Airways. This makes things very different from the U.S. where one's old family home in Europe became an imaginable tourist destination from early on in the 20th century. Prewar Japan could only envision Europe in terms of its exported culture, and unless if you are a big fan of dense philosophy or old imperial history, this meant the visual splendor of the Parisian avant garde.
After 1945, Japan had no air travel for some time. Ordinary citizens were not permitted to travel abroad for leisure until April 1964 (little-known but true), and even business travel was highly restricted until 1963. In the mid-1960s, then, being able to go to Europe became the ultimate status symbol for wealthy, educated Japanese -- and I can only guess that Paris would be the most brag-worthy place to visit.
This is one of those things that are so obivious to a native Japanese, but hard to write a post that could qualify as a decent answer in this subreddit.
Fascination with and idolization of French culture among the Japanese public surely happened through pop culture, most notably through film and music, in the pre-WWII era.
This is a list of "Top 10 Movies of the Year" published by a Japanese cinephile magazine, Kinema Junpou, I cited elsewhere to show that American movies were popular in pre-WWII era:
http://wonderland02.web.fc2.com/movie/cinema/cinemabest01.html
From a cursory look, this list is dominated by American movies, and then German, but I find some French films too. The best artistic film of 1925 was "La galerie des monstres" (1924) by Jaque Catelain, #5 film of 1926 was "La roue" (1923) by Abel Gance, #9 of 1927 was "Carmen" (1926) by Jacques Feyder, #2 of 1931 was "À nous la liberté" (1931) by René Clair, #4 of 1931 was "Le Million" (1931) by René Clair, #1 of 1932 was "À nous la liberté" (1931) by René Clair, #2 of 1933 was "Quatorze Juillet" (1933) by René Clair.
Probably the last film "Quatorze Juillet" is very relevant to this thread because it was released in Japan under the Japanese title "Pari Sai" (Paris festival). It suggests that idolization of Paris was already there among people responsible for importing this film. This film became a big hit and the Japanese still call this day (Quatorze juillet) "Pari Sai" in Japanese. It has still a very exotic, romantic tone to it.
I should also point out that French film was very popular post-WWII, at least up to Nouvelle Vague.
As for music, in pre-WWII era, European influence was stronger than American, and French pop music was very popular. In Japan, there is a music genre called "shanson" (obviously from 'chanson') which denotes French-style music up to around 1960s. On this Wikipedia page "Shanson" , the first table shows some French songs that have been translated to Japanese and covered by many Japanese Shanson singers.
This Google Image search for "Prewar Shanson" might help you get an idea of what Shanson meant for the pre-WWII Japanese. Shanson as a music genre has lost its momentum for long, but when you see this Google Image search for "Shanson", you can see that the fascination is still there.
Now, please note that the following is my personal speculation and not backed by facts.
As is well known, postwar Japan was bombarded by American culture. In that climate, French and other European cultures (German and Italian in particular) became to feel old and obsolete, and people sort of stopped updating/renewing the image of those countries. To an average Japanese, New York is always a current cutting-edge city. On the other hand, Paris is still an object of a vague nostalgic feeling, a reminder of the good old days. And IMO that feeling among the Japanese public has its roots in pop culture of the early 20th centry as I showed above.