Sometimes it is best not to say I love you to someone you love because it never measure up to how you really love that person. It more than just words typed in your message or said that pleasing the ears, it is even more than the feeling that is supposed to entails. The person being said to never really feel the same 'love' that you are feeling. He/she would probably just reply 'i love you too' because that they are supposed to do.
At this point you don't even want the reply because it is NOT the same. It is the love that you do not want to tell because it is just for you to know how you really really truly love the person and pray to God that 'i really love that person more than words can ever describe. Take care of him/her for me.' Therefore, it is sincere.