Although I cannot comment on the meaning of "ai" to Japanese, I will say that there are four Greek words for "love": "eros", "stergo", "philo", and "agape". Each of those words have a distinct connotation, and all four of their connotations are encompassed by the English word "love". In addition, all four of those words are still in use in the English language. Since the evolution of our language has infused the word "love" with such broad scope, it is often one of the most misinterpreted words in the English language. Being specific about the type of love you intend by using non-generic terminology gives you the ability to avoid such misunderstanding, but limits your ability to convey deeper meaning through subtext, intonation, and other subtleties.