The words "I love You" carry enough meaning to flood the Earth. Love is what we live for and love is what we die for even if there is a lack of it. Life and I mean everyday life is centered upon the way you love somone, the way you don't don't someone or the way you love or don't love yourself.

Why is it never okay to tell someone you love them after knowing them for week? Or even a day.

I say go for it! Feelings are like the waves of the ocean anyways. You can never be sure if your rising or falling, so why not tell you that I love you if I truly love you. Even if only for that moment when the words leave my lips I trully mean it.
Even if tomorrow I will change my mind, who's to say that I won't change it again? There is no definite love so why should there be a moment designated in life when I can tell you that I love you?
I may not live to see tommorrow so if I love you today than that should be all that matters.