In an age when advances in technology and industry and increasing in population have significantly reduced human's pleasure and happiness and imposed various stresses and pressures, which all in turn have caused high rate of depression, investigating and analyzing the concept of happiness and introducing its reasons in the views of Iranian's poets can help to spread and deepen happy life, joy and mental contentment.Although, language and mind of Iranian poets have, in various ages, propagated the word “happiness” and its synonyms, which are briefly discussed here, the present paper focuses on the common reasons for happiness from the viewpoints of three Iranian poets who had completely different thinking fundamentals. Khayyam is not considered a real mystic, while Molavi is true mystic and Nezami is really different from Molavi, but not so much from Khayyam. Since all of them are among the few prominent persons who have broken the limits of ordinary beliefs and by dynamic opinions and thoughts and complete understanding, have discussed main humanity problems, the author, regardless of the signet of sage or mystic on forehead of each of them, has analyzed the similar points in their works. Enjoying comparative analysis, the common factors of happiness are investigated in the poems of Khayyam, Molavi and Nezami.