Concepts can vary from specific to abstract, and it makes sense to have more concepts, at varying levels of abstraction, on topics we care more about. Hence the myth that Eskimos have more words for snow.
Our relations with each other are very important to us, and they vary in a great many important ways. Why then do we use the word "love" so often to describe our relations, as in the famous three words "I love you." Why not instead use a variety of more precise words that convey more detailed meaning? Why not say "I wistfully-romantically-heart you" or "I hopefully-lustfully-want you" or "I wearily-unwillingly-stick-to you"?
The answer comes, I think from realizing that if we described our relations in more detail, we would have to acknowledge finer changes in our relations. Our current "I love you" approach lets us use the same descriptor at all stages in our relation, and at all points in our mood cycles. We don't have to announce when our relation moves from hopeful lust to wild passion to tender comfort to favorite-old-shirt familiarity. Such announcements could be quite awkward, especially if our perceptions are not exactly in sync.
I suspect we are also purposely vague with many of the other words we use, but I haven't spend much time trying to think of other examples. Can readers think of more examples?
Added: Tyler once listed many different reasons to say "I love you."