The pessimist versus the optimist:
Some people have a natural tendency towards pessimism, while others towards optimism. Interestingly, often those who have a good life with many blessings and no true suffering, have a pessimistic outlook at life in general and to their own predicament, in particular. They expose all the “bad” that they experience to the forefront of their mind, and bury the blessing that they receive. Every occurrence that does not meet their expectation, suddenly becomes a tragedy in their mind, and they never have ability to enjoy the blessings that G-d has granted them. However, the optimist, although he quite often experiences real suffering in his life, he knows how to see above and beyond it, trusting in G-d, and focuses on the few blessings that he does have. These two dispositions create a strange phenomenon in which the rich pessimist is a sad and bitter person, while the suffering filled optimist is cheerful and jolly. Every person has ability to work on his character and train himself in positive thinking, in how to become an optimist. The Torah commands a Jew to praise Hashem for every breath that he breaths, and to serve Hashem with joy and hence one is obligated to strive to look at the good that G-d grants him and not embellish the bad, or create an issue of everything that does not flow their way. The Zohar teaches us a great instruction in this regard, which is that the attitude of the person actually has power to affect the blessings that he receives from above. Thus, a sad and bitter fellow who is always pondering the negative and puts on a morbid face, will himself sabotage any positive blessing coming from above, [as just as he thinks of the negative, so does the Divine Sefiros who judge his soul]. However, one who is happy and focuses on the blessings of his life, draws down positive interaction from above, and merits even more Divine blessing. So not only do we have ability to banish the bitterness and live a happier life through changing our focus and attitude, but we even have ability to affect that G-d truly grants us blessing, and remove the causes of potential bitterness from occurring.
 Based on Igros Kodesh 11:321
 Igros Kodesh 11:383
 Zohar 2 184b