Yes! My "I love you" was to the only one which was special to me. It is ages ago though! haha! I'm not sure if I used "could/would" propperly, auxilialy verbs, for "could/would die for you", but I guess the writer meant something like "I could die for you (If I really needed to)" -> "I think you as the one much more precious than myself" -> "I love you". However, the writer might have been just a kind of a little self-deprecation, as you might felt. His name was FUTABATEI SHIMEI and it is said that it was from "KUTABATTE SHIMAE" which means something like "f___ you". He's a very famous and grate writer here, though, I'm doubting his sense of naming unless it was his witty joke! :-P As for "Ai no kokuhaku"(I'll put "ai no - about love" in order to supecify what the kokuhaku is about) and confession, in my understanding, this is to state what only you know but others don't(, though some tend to be aware of it) So there would be a case the other don't accept your "ai no kokuhaku". Does the same happen to "I'm in love with you"? I thought it is a phrase for people　who have already known each feeling because the word "with" which is related to "together" for me.

HAHA! That bit about your dog was hilarious :P And yes, "I am in love with you" is only said to humans (unless the speaker is a creep and needs some mental help). But more than that, you can only say it to your "romantic significant other" So you wouldn't say it to your friends or family. Actually, it's not too different from kokuhaku... It is a confession that could completely change the relationship of the two people involved. It much more romantic, personal, and deep than kokuhaku though, I think. That's interesting that a write would translate it to "I would die for you" because to me, "I love you" and "I would die for you" are completely different! (Not that I have a death wish) I would sacrifice my life for anyone I think is more necessary for the continuation and evolution of human kind :P (I am of course biased in that judgement). I would die for many more people than I love... I hope that doesn't sound depressing, hahah!

Thanks for your comment, Jasmin. Ah yes, you have another phrase "I am in love with you". Is it to only a human? I guess my "I love you" said only a few times meant that one. However, it wasn't still my phrase then, really. I remember that It didn't convey to myself much, though I meant to. Another old writer translated it to "I could die for you", I suppose. I heard there were quite a few writers were in trouble to translate because we don't have the idea. For me, It might be "Thank you for being by me and I'll support you dearly all the time" and this is exactly what I'm telling my dog sleeping these days. haha.

I think I love you is a very powerful phrase that is way too overused in America these days. :P It's become so commonplace and without much meaning that the English I love you is equivalent to the Japanese I like you! :P I found that extremely interesting when I first started studying Japanese. On a daily basis I say I love you to my friends, my dog, my computer, the internet, etc. haha! "I am in love with you" carries a much stronger message than "I love you" here, and "I am in love with you" is much much much less common much more personal to say.