Having lived in Japan and came across to many Japanese who live / lived outside Japan, apparently saying I love you is really a big deal for Japanese.

In fact, it is a big deal for people from Asia in general. This is mainly because of the cultural difference and being "conservative" about "opposite sex" is considered a good moral.
For example, segregating male and female in junior high school and high school is very common in Japan. All boys or girls school can be found at ease and most of the prestigious are in fact boys or girls only school (there are exception ofcourse). Even at the mixed sex school, finding a couple who proclaim they are dating could be as difficult as finding Willy (in case you are not familiar with the book, check this out).

On the other hand, the influence of western culture cannot be ignored. According to these posts
younger people in the western Japan are starting to use I love you to each other, and kids in elementary school think "I love you" is basically adult version of "I like you". 


As it has been mentioned in the post "How to say "I love you" in Japanese?", the phrase "I like you" has been the "I love you" in Japanese. But as time goes by, who knows, one day I love you might be used in Japan as lightly as the way those Hollywood starts say to each other.