Question: the Gemara says at the first " : " R' Yochonon said B'shem R' Yosi Ben Zoma ... Anyone who depends on his own merit ... They make it dependent upon the merit of others. So we have a " T'aina " (complaint) against Chizkiyahu for not depending on someone else like Moshe Rabeinu did. But The Gemara said earlier that Dovid Hamelech Davened to Hash-m that he should watch him and he said " KI CHASID ANI" showing that he was depending on his own merit and not not the merit of others?
David ha'Melech called himself a "Chasid." The word "Chasid," and the concept of "Chasidus," always connote going beyond the letter of the law and doing more than what is required. David ha'Melech was a "Chasid" in the sense that although he was the king of Israel, he still woke up before sunrise, and he was involved in examining bloodstains to rule on questions of Nidah -- very unkingly acts.
When a person's merits supersede those of his ancestors, he may make claims based on his own merit, since he cannot rely on his ancestor for such merits. Since David's fathers were not kings, they could not possibly lay claim to these acts of Chasidus. Chizkiyahu, on the other hand, did not claim to have reached a level higher than his ancestors in righteousness. He prayed that Hash-m should answer him since he went in the ways of Hash-m etc. Therefore, he was not entitled to make claims based on his own merit, since he could just as well have mentioned his fathers.
Be well,
Rabbi Mordecai Kornfeld