Bertrand Russel writes about obsessive love (in relation to Fitzgerald and Nabokov), and it reminds me of a previous thought I'd had: that no man is complete without some unobtainable love.
Those two authors write mostly about obsessive romantic love (or lust), but their characters stand for far more than mere sexuality (or even humanity). All men and women need an object of desire -- moral, spiritual, philisophical, material? -- to yearn for and strive after, knowing it can never be obtained. This is the essence of tragedy, and the foundation of greatness.
No one accomplishes anything great by aiming at the attainable. Greatness is achieved in incremental steps, to be sure, but the ultimate goal must stand forever out of reach or it's not even worth the effort. Greatness springs from tragedy; tragedy puts the accomplishments of life into scale, and reveals their greatness.
As a banal example, consider the SAT. If everyone received perfect scores, what significance would the test have? It would tell us nothing about anyone's abilities, intelligence, knowledge, or determination. Tragedy serves the same purpose; by highlighting the failures and disappointments of life, success can be elevated to the level of greatness. By striving for impossible goals, through obsession with the unattainable, a man is stretched to his fullest extent and his greatness can be rightly judged.