Neuroscientists confirm that each time we repeat an action or thought, the groove in the brain corresponding to that action or thought deepens. This fact can serve us both positively (look both ways before you cross the street) or negatively ("I will never be able to...." fill in the blank).
Remarkably, scientists have discovered that our brains have neuroplasticity, which means that they are far more mutable than we have thought. This means we are able to influence physical change in the brain in response to our thoughts.
Shifting our thoughts from what we lack to being grateful for what we have is profound. From, The Art of Personality, Inayat Khan:
"Gratefulness in the character is like fragrance in the flower. A person, however learned and qualified in his life's work, in whom gratefulness is absent, is devoid of that beauty of character which makes personality fragrant. If we answer every little deed of kindness with appreciation, we develop in our nature the spirit of gratefulness..."
"There is much in life that we can be grateful for, in spite of all the difficulties and troubles of life. Sadi says, 'The sun and the moon and the rain and clouds, all are busy to prepare your food for you, and it is unfair indeed if you do not appreciate it in thanksgiving.'"
"...But little actions of kindness, which we receive from those around us, we can know, and we can be thankful if we want to be. In this way, man develops gratefulness in his nature, and expresses it in his thought, speech and action as an exquisite form of beauty."
I especially think this following section is important, in how we express our gratitude and how it may or may not be reflected back to us:
"As long as one weighs and measures and say, 'What I have done for you' and 'What have you done for me', 'How kind I have been to you' and "How good you have been to me', one wastes one's time disputing over something which is inexpressible in words; besides, one closes by this that fountain of beauty which rises from the depth of one's heart. The first lesson that we can learn in the path of thankfulness is to forget absolutely what we do for another, and to remember only what the other person has done for us."
"Throughout the whole journey in the spiritual path, the main thing to be accomplished is the forgetting of our false ego, so that in this way we may arrive some day at the realization of that Being whom we call God."
So, be grateful every day. Say aloud five things you appreciate about your life every morning and evening. Express sincerely your gratitude towards others, to your beloved, your colleagues, your friends, your neighbors. Expect nothing in return (although I suspect this practice may garner a few smiles).
Then watch: the fountain of beauty which rises from the depths of our hearts, where we feel our deepest emotions, have sympathy for others, and where we can connect to the Universal Heart, see what perfume begins to permeate your being.