I was raised in a home where whenever we left the house, ended a phone call or went to bed we told our parents we loved them. Were they forgetful? Have an egotistical need to hear it all the time? Didn’t they know? Of course they did. While it was automatic I can’t imagine what it's absence would have meant. It would have had to signify something earth shattering. What did it accomplish? It represented the relationship we had, making sure we never took it for granted.
Why do we have to tell G-d we love him if he knows already? If we know it and he knows it can’t we just call it a day? The more we tell G-d we love him the more conscious it becomes and the more aware we are of our relationship. Remaining passive, making no effort to communicate with G-d weakens the relationship. Expressing our love for G-d isn't for him. It’s to ensure that our relationship remains in the forefront of our minds and we don’t put it on the back burner figuring its good enough to have recognized it once or twice.
I think another reason we need to express our love for G-d, is to acknowledge all he does for us. When you are given a gift you should acknowledge the giver. G-d's commandment that we love him and express that love is the actualization of our bond with him.
Wait, what does "express" mean? We tell him, right? Actually, it is through our actions that we tell G-d we love him. Does he really need us to let him know by running around carrying out all those commandments he gave us? What does he get out it besides pride in seeing his children develop the character traits that bring us closer to him and each other? So though he knows exactly how we feel, it’s not enough to love him, or to say we love him, now we have to show it too? By fulfilling his commandments we not only strengthen our relationship, but we work on perfecting ourselves and the world he put into our hands to care for elevating ourselves and everything around us.
Like a devoted parent who wants the best for his children, G‑d trains us to express our love for him for our benefit. G-d doesn’t need us to express it so much as we need to do so. Because everything we have is his gift to us. When we fail to express our love for the creator who loved us enough to give us life, his words and guidance, his commandments and world, we fail to fully recognize our relationship to him and that we are literally a part of him. And what could possibly be greater than knowing we are a part of G-d above?