I think that telling someone "I love you" is like saying "you are a wonderful, precious human being to me". Therefore, if we feel loved, we feel that someone recognizes and confirms our value as a complete being. The expression of love is, in this sense, the most pleasant outside confirmation of our value that we can get.

With regards to love and sex, I believe that they can (and it's most enjoyable that way), but don't have to be connected (they can be enjoyable even separately).

Cheers! : )