ויצא יעקב מבאר שבע וילך חרנה ... (כז-לג)
Rashi tells us that when a tzaddik leaves a place, it makes an impression (רושם), “for while the righteous man is in the city, he is its beauty, he is its splendor, he is its majesty. When he departs from the city, its beauty has departed, its splendor has departed, its majesty has departed.” The Medrash comments that only when a tzaddik leaves a city is he missed; when he’s in the city, however, he is not appreciated. Unfortunately, this a very common phenomenon - people do not appreciate what they have until it is gone!
The Hebrew word for appreciation is translated literally as "הערכה" but when we discuss the middah of thankfulness, we call it "הכרת הטוב" because the essence of this middah is to recognize the good that one has within him. Just being a good person alone does not mean that you will be appreciative. There are many people who have alot; and yet they are very poor! There are those who have little; and yet they are extremely wealthy. Why are they wealthy? Because they appreciate whatever they have. The value of appreciation is what makes one a wealthy person.
In truth, the essence of a Jew and his connection to his Creator, is dependent on this middah of Hakaras Hatov. True Hakaras Hatov is recognition (Hakaras) of the One and only Source of Goodness (Hatov) - which is Hashem. The more a person sees Hashem in his life, the more appreciation he will feel. For this reason, a Jew is called a “Yehudi” (יהודי) and the very first words that he speaks when he wakes up in the morning is “Modeh Ani.” (מודה).
R’ Shimshon Pincus zt”l points out that we say “Modeh Ani” and not the more grammatically correct “Ani Modeh” because the first word that a Jew should pronounce upon opening his eyes should not be “Ani” - making it all about himself, but rather a way to say thank you to Hashem for giving him back his life each morning.