"Whenever you want to cheer yourself up, consider the good qualities of your companions." We turn to Marcus Aurelius for the realistic wisdom of stoicism, but he has another gift to offer us. What's striking--and rarely discussed--about his Meditations (free online text) is the nature of the first chapter of the book. Book One is a lengthy and heartfelt expression of gratitude to those who taught and nurtured him. It begins with these lines: "From my grandfather Verus I learned good morals and the government of my temper. From the reputation and remembrance of my father, modesty and a manly character. From my mother, piety and beneficence, and abstinence, not only from evil deeds, but even from evil thoughts; and further, simplicity in my way of living, far removed from the habits of the rich."
Marcus Aurelius had some advice for people who wanted to show gratitude and improve their outlook on life. You can read some of them here.