Gemara :


Ein Ayah: There are people who are naturally blessed with good qualities and who do not have to work hard to follow good paths. A person like that will not attribute his shleimut (completeness) to himself but will attribute it to his forefathers, who passed on these traits to him. Someone whose qualities are not naturally the finest but who worked hard to acquire good attributes will normally attribute the traits to himself, for he toiled until he arrived at his proper state. However, the truth is that one who was born with less than ideal characteristics still must have inborn strength, hidden from earlier generations, which enable him to overcome his bad traits. This is along the lines of the Kuzari, who says that a special quality can disappear in a rasha’s (a wicked person’s) personality and reappear in the rasha’s righteous son’s personality. Therefore, in that case, one can still attribute his success to others. In contrast, someone who was born with precious qualities still will usually apply himself to follow the ways of Hashem by doing good deeds beyond those for which he was naturally prepared.
Chizkiya attributed the merit to himself because he was the son of a rasha and, therefore, he did not think he could attribute his acquisition of shleimut to inheritance from his forefathers. In truth, the success could be attributed to others, as Chizkiya was told that he had a lot of help in overcome shortcomings from the hidden special qualities that could be traced back all the way to David. These positive qualities remained inactive in his father, Achaz, but reappeared in Chizkiya. This idea finds expression in the gemara’s previous statement that Chizkiya saw in the Divine Spirit that bad offspring would come from him. This is because he was concerned with bad attributes that he was born with and saw how these attributes were actually going to play out.
Moshe attributed the merit to others because he was born with good and holy qualities from holy, pious parents in an unbroken chain from the "fathers of the world." His success was attributed to him because he exceeded drastically the expectations from the attributes with which he was born. That which people say that he was born with exceptional qualities is contradicted by Chazal and the simple reading of the pasuk, "She saw that he was good," and from straight logic. Certainly the greatest person ever created had very fine natural attributes, but he still added on a tremendous amount of shleimut above and beyond what he naturally received.