שאלמלא בא מלאך" - Sanhedrin 92b - אמת ואמונה
יוסטרו על פיו, ביקש לגנות כל שירות ותשבחות שאמר דוד בספר תהלים Nevuchadnetzar was singing שירות ותשבחות to Hakodosh Boruch Hu. A Malach came and hit him on the mouth and he stopped. Had the Malach not come and hit him, his ותשבחות would have surpassed the great שירות ותשבחות that Dovid Hamelech said in his Sefer Tehillim. How are we to understand this Gemara? Nevuchadnetzar was singing שירות ותשבחות to Hakodosh Boruch Hu and the Malach came and made him stop? This does not seem to be fair. If his singing Hashem's praises would have surpassed Dovid Hamelech's, then so be it. We need to understand what happened here. Dovid Hamelech sang praise to Hakodosh Boruch Hu all day and all night, in all situations; good and bad. When Dovid Hamelech had the worst Yisurin, was being hunted down by his own son, he still attached himself to Hakodosh Boruch Hu and sang Shira to Him. Nevuchadnetzar was singing Shira to Hakodosh Boruch Hu when he was on top of the world. He ruled the world down here, and life was good for him - there was nothing in matters of this world that he was lacking. Therefore, the Malach came and hit him to make life not so good for him. This was done to see if even when things were not so good would he sing praise to Hakodosh Boruch Hu, like Dovid Hamelech. Nevuchadnetzar was not held back from singing praise, he was just tested to see if he would still do so. As soon as was hit, and everything wasn't good, he immediately stopped singing praise to Hashem.