Daf Yomi - Taanis 25 - the Ohr Sameiach on nacheim and nachamu

From Reb Chaim B.

The Rosh asks: why is it that we only recite nacheim in the shmoneh esrei of mincha on 9 Av - if it is akin to "me'ein hame'ora", a tefila based on the character of the day, we should recite it in all our tefilos just as we recite ya'aleh v'yavo in every tefila of rosh chodesh and al hanisim in every tefila of chanuka? The Ohr Sameiach (hil tefila) answers this question with a gemara in Ta'anis (25). Shmuel haKatan declared a fast, and after a full day of tefila it began to rain. The tzibur assumed this was due to their good grace, but Shmuel haKatan warned otherwise. He gave a mashal to a Master whose servant needs a certain request granted - rather than grant the favor immediatly, the Master let the servant sweat out the day in suffering and pleading and only then consented. Explains the Ohr Sameiach: if we were to say nacheim at night and again in the morning, and then follow with the leining of "ki tolid banim" and "asof asifeim", it would seem that Hashem is piling suffering on before granting our wish. By not saying nacheim until mincha at the end of the day, we go immediatly from the close of our tefila of 9 Av of "nacheim" to shabbos nachamu, where Hashem offers the response of "nachamu nachamu ami".

(Look at the comments there for another nice answer.)