Because English sounds cool even in Japanese music :)

I'm not a japanese person, but I think I kind of know why this happens. Actually, this doesn't only happen in Japanese songs, so does in Chinese songs. Sometimes,when I listen to a Chinese song, I may come across some English words, like the most common ones--I love you, honey, darling, baby, babe....some words that are related to love or pretty girls I guess this is because English is the most prevailing language in the world, and many other languages more or less borrow her words And because of her popularity, it's normal to quote some words or sentences in songs. I guess, if the international language is now Chinese, or French or whatever that language is, many songs will use its words.. Another example, Korean modern popular culture is quite famous in Asia, and so many other singers from other Asian countries may use the sentence"Sa rang hae yo"(which means I love you)..like there's one Vietnamese song(I've forgotten the name), you can find the sentence"sa rang hae yo"in it, and there's sa rang hae yo in one mandarin song... I think people like to chase after new things, or popular things

In my opinion, most Japanese tend to use English especially when expressing feelings not only because of shyness but also because it is easier and shorter to say compared to its Japanese counterpart.