The phrase "I love you" functions in a strange way. One person says it to another to convey affection and desire. "I love you" means only that "I find you appealing" or "I feel happy when you are near" or something similar. It is an expression of opinion and emotion and has an important role to play in the amorous relationship.
However, there is another side of "I love you" that is unspoken and perhaps not widely considered. It should be considered as an observation of action, rather than an expression of feeling. The lover who sets aside a promotion at work so that they can look after the beloved has performed an act of love. The lover who cares for the sick beloved by preparing meals, buying medicine, etc., has performed acts of love. In this way, the phrase should be said as "I loved you" or even better, "I loved you today."
Love, after all, is no less than choice that has become action. Love, if limited only to a state of emotion, is fickle and representative only of response to another. There is a place for that kind of love in the world, but by itself will not overcome difficulties and evils.
Perhaps the pinnacle of "I love you" is to say the subjective opposite. That is, "You love me." This is not a selfish or narcissistic statement, but rather one of humility. Occasionally one might hear it in the form, "Thank you for loving me" but this is gratitude. To say to someone, "You love me" or even "you loved me" is not a measure of ego or self-aggrandisement. It is an observation of the selfless acts of another, directed wholly towards the beloved.