Somebody asked me the difference between ai, koi, and suki/daisuki, words commonly rendered as “love” in English, especially in anime and manga contexts. Here’s the short version I came up with:
You can suki pizza, you can ai your mom, but unless you're willing to stick your tongue in their mouth, don't say koi.
The more detailed explanation is that suki is simply typical Japanese cultural reserve and indirection. Suki: “X is liked (by me)”, daisuki: “X is liked a lot (by me)”. The difference between ai and koi is an instance of a general rule: if you have two words that appear to mean the same thing, where one of them is a Chinese-derived on-reading and the other is a native Japanese kun-reading, then the Chinese term is more objective, formal, and/or polite; the Japanese term is more subjective, personal, and/or casual, often describing feelings rather than reality. The historical basis for this is that the Chinese terms were imported and used by the upper classes and the government.
It’s easy to find a lot of examples where the two share the same kanji, but even when they’re different, like ai and koi, the rule holds up pretty well. Ai is your loving relationship to a living being, koi is your passionate feeling about someone.