This article is from the Japanese FAQ, by TANAKA Tomoyuki tanaka@cs.indiana.edu and Olaf Meeuwissen olaf@IMSL.shinshu-u.ac.jp with numerous contributions by others.
You don't! At least, according to common lore. Most people sug-
gest you show the person in question your affection. But, if you
want to ignore this advice, you can try your luck with "ai shite
imasu" or "daisuki desu". Since you (usually) say this to some-
one you know (quite) well, it is probably more natural to drop
the politeness level a bit and use "ai shiteru" or "daisuki da"
instead. To add some emphasis, you could add a sentence final
"yo".
 
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