"I love you."
Some people say those are the three most beautiful words in the whole universe, and I partially agree.
"Now kids, there are three kinds of love. First, there's the love between children and parents, and I think you're already familiar with it. Second, there's the love between men and women, and it's too early to tell to you about it. And third, there's the People's love of The Communist Party, and that's what I'm about to tell to you now..." - a Soviet Union sex education brochure (according to a joke, of course *g* - this one appeared in Finland)
"I love you" has so many different meanings in different situations. Maybe this is because love is such a relative term. For example, I can tell you I love you; I like Everything a lot, it's a wonderful place and there are a lot of wonderful noders in here. Yes, I like this place, even with the tr0llz. I love the Usenet, even when some might say it's just a hideous hellhole of flamers and spam. I love my country too, at least the best parts of it. I love this whole world.
And I have known people to whom I could have said these words without any doubt. And even then I've had to say that in different meanings; I have said it to Cool Persons I Really Really Respect; and (often as followup to the previous one) I've said it because of true emotional love. (No, I haven't said that to anyone to get them to bed. Honest. I usually mean what I say, these kind of things I cannot afford to toy with. =)
All I can say is that if you feel that way, that someone or something messes your thoughts positively, don't be afraid to say it! Love has many manifestations (and I'm not referring to Marxism here, no pun intended =) and admitting it to the loved one is always a step to the right direction.