If hearing the leader counts, why does everyone answer?
The Gemara in Sukkah says hearing counts. It learns that a listener is like someone who answers. Then it records that the Sages ruled a person who heard and did not answer has still done what is required.
So why answer? The same Gemara calls answering the opening verses a mitzvah. Rashi's explanation is that Hallel was first set up as one person reading and everyone else answering a little, because not everyone knew it. By Rashi's day people could read all of Hallel themselves, but the answers stayed. Rashi says they were kept so the first rule would not be forgotten.
The Mishnah Berurah (a central commentary on daily Jewish law, written around 1900) says the congregation can fulfill these verses by hearing the leader. But it adds that it is better to say them quietly yourself. Its reason is simple: sometimes a listener is not paying attention.
The Magen Avraham (a 1600s commentary on the Shulchan Aruch, the main code of Jewish law) adds a condition. You have to actually hear the leader. If you missed "Yomar na Yisrael," you did not fulfill it. His advice is the same: say it yourself, quietly. Kaf HaChayim (a large commentary written in Jerusalem in the early 1900s) adds a second reason. Sometimes the leader stretches the words with a long tune.
Kaf HaChayim also quotes a complaint about a common mistake. The verse is often said only up to "Yomar na Yisrael," without finishing "ki le'olam chasdo." That leaves half a verse.
These sources agree on the main point. Hearing works, and saying it yourself is better.
Sukkah 38b,Rashi on Sukkah 38b,Mishnah Berurah 422:20,Magen Avraham 422:8, as quoted in Kaf HaChayim, Orach Chaim 422:44Israel already includes everyone. Why name three groups?
The verses call on three groups to say "His kindness is forever": Israel, the house of Aharon (the priests), and "those who fear Hashem." The priests and the God-fearers are part of Israel, so the list looks repetitive. The sources give two kinds of answer.
The first is about proof. Midrash Tehillim says it is not fitting for a person to call himself good unless others vouch for it. So the psalm calls witnesses. Israel can speak because it lived through God's kindness. The house of Aharon comes next, and the Midrash gives a picture. A landowner treats his tenant farmers well, but they do not see how he runs his own house. Who does? His servants. God's servants, it says, are the priests, who serve before Him all the time.
The second answer is that each group has its own reason. Radak says our teachers disagree about whether the psalm is about David or about the days of the Messiah. Reading it as David, each group has its own reason to thank God for his kingship. Israel is safe from its enemies. The priests had lost their city, Nov, to Shaul's sword. Those who fear Hashem now have peace and time for learning, and David says he is one of them.
Midrash Tehillim 118:4,Radak on Tehillim 118:1, 118:3 and 118:4,Shmuel I 22:19Nothing specific happened. What am I thanking God for?
Most thank-yous name a favor. This one gives a reason instead: "for He is good." The verses do not say what God did. Meiri and Radak tie the psalm to David's kingship, or to the days of the Messiah, but the four verses name no event.
Midrash Tehillim answers with a parable. A man in town lends everyone what they need, and the townspeople speak well of him, though he charges interest. Then someone offers to lend for less, and they thank him instead. Then someone offers to lend for nothing. That man deserves thanks. The Midrash says God feeds His creatures for nothing, so thanking Him fits.
The Gemara adds a number. This same line opens Tehillim 136, part of what the Gemara calls the Great Hallel. That psalm has 26 verses, and each ends with "His kindness is forever." Rabbi Yehoshua ben Levi says the 26 stand for the 26 generations from Adam to Moshe. God made them before He gave the Torah, and fed them anyway, out of kindness. Rabbi Yochanan says it is called the Great Hallel because God sits on high and gives food to every creature.
And "forever"? Midrash Tehillim says God does not do kindness for a year or two, but always. It points to the Torah, which it calls chesed, kindness, and says it was given as an inheritance and never taken back. It also quotes Eichah: "The kindnesses of Hashem have not ended."
That is the Midrash's answer, and it is a parable, not a proof. The verse itself says only this: He is good, and the kindness lasts.
Midrash Tehillim 118:3,Pesachim 118a,Meiri and Radak on Tehillim 118:1,Eichah 3:22