Commentary for Taanit 45:7

Rain of [good]will, blessing, and generosity, etc.: Meaning to say, of Your will - as it is said, "May it be Your will, etc."; and that it should also be the will of Israel, such that it not fall furiously. And blessing is that it be a blessing for You and a blessing for us. And [generosity] is that which they said in the blessing for rain, "and generosity for You" - that it not be by way of an obligation. And that it be generosity (nedevah) for us - as it stated (Psalms 54:8), "Then I will sacrifice to You with a freewill sacrifice (nedevah); I will praise Your name, Lord, for it is good, etc." That is why he brought a bull of hodaah (thanksgiving).

Over an excess of good, etc. Bring me a bull, etc.: It appears that it can be explained that the reason of the thing, that we do not pray over an excess of good - even though this good of excess rain is bad, as I wrote above, since it weakens the land, so it doesn't produce fruit - because it is nevertheless like ingratitude. For at the beginning, he prayed and requested rain, which is good for the world. Yet he said, "Bring me a bull of hodaah," upon which to confess [sins], as Rashi explained. That is to confess that he did not act properly, by requesting the stopping of excess good. But "Israel, whom You brought out of Egypt" - he made the thing depend upon His taking them out of Egypt. As You grew angry with them [and] they could not withstand [it] - for example, the trials of water and Mei Merivah. [And] they were not able to withstand the flow of excess good to them - for example, with the manna and the quail. They were not able to withstand [it], as it is stated about it (Numbers 11:33), "The meat was still between their teeth, etc. when the anger of the Lord flared up, etc."