Many times we say "thank you" or "you're great" to people we feel have done something good or at a moment when we need to express our appreciation for them, but sometimes these words are not enough. You may want to tell a certain person how special he is or how important you think he is, but the words "thank you" or even "you're great" are not enough to express your true feelings.
People in general need to hear kind and more expressive words to feel that their efforts were not wasted, or that they are really special in the eyes of the people around them. A nice word or compliment has a great impact on people. Sometimes a simple word can make a difference to someone more than you can imagine. For example, if someone helped you with something small or even stood by you in a difficult time, "thank you" is not enough, you have to show him that you fully appreciate the situation and this help.
Sometimes, we may need to use different words or add more details to clarify our feelings more clearly. You can say, "You're more than wonderful, things are really easy with you," or "I'm lucky to know you." These words will make the person feel more important, and that he or she has done something truly special.
What I want to say is simply that the words "thank you" or "you're good" are not always enough if the person has really had a positive impact on your life. Sometimes, the compliment needs to be greater to have a real impact on the person in front of you.