How to say: I love you
In the West we use the word 'love' with zero afterthought. "I love your shoes", "I loved that marinara". The Japanese however do not express such emotion. They would simply say: 'I like your shoes', or 'I really liked that marinara'. But love? No no! That is a word almost never uttered by a Japanese.
What about loved ones?
To a love one in Japan you will say: 'suki' (like) or 'dai suki' (really like). That is standard. Even new lovers, neck deep in infatuation won't say: I love you.
On the phone before you hang up, you will hear a spouse or a partner say "suuuki".
How do you say it though?
Literally that means: I am loving you' ('you' is implied, and so is 'I'). It is interesting to note that they use the present progressive tense to communicate a continuous condition. We do the same thing in English with certain words.
Why don't they say it?
Interestingly, if you ask that question to enough Japanese you'll start to see a pattern of responses. A common reason is that, according to their thinking: you show someone that you love them by actions and not by words.
This is quite interesting, and in a way, quite wholesome. However, there are flaws in this reasoning. If you follow up, and ask: "well how do you show someone you love them", they will respond: "by working hard at your job, or taking care of the house".
For all you know, they are working hard at their job, day and night because they can't stand being around their spouse!
Clearly, the best way two show would be a balance of actions and words.