Rashi: He composed it with Ru'ach ha'Kodesh; he saw that his son Shlomo will build the Beis ha'Mikdash, and on that day he will marry Bas Pharaoh. About this it says "Ki Al Api v'Al Chamasi Haysah ha'Ir ha'Zos l'Min ha'Yom Asher Banu Osah" (Yirmeyah 32:31). Therefore, he said this Mizmor.

Radak #1: "Li'Shlomo" means for the sake of Shlomo. This is like "Imri Li Achi Hu" (Bereishis 20:13). David intended to build the Beis ha'Mikdash, and he gathered silver, gold and copper for it, but he did not reap the fruits of his toil; rather, his son built it. Also other people do not fulfill their intents.

Radak #2: There is a hint to Melech ha'Mashi'ach; also he is called Shlomo in Shir ha'Shirim (3:11). First it discusses Shlomo.

Malbim: It teaches that man's efforts do not help at all; all depends on the counsel of the supreme Hashgachah. The next Mizmor discusses when efforts help.

Rashi: Why build the Bayis and veer away from Hashem (refer to 127:1:1:1)?! Since He does not want it, "those who built it toiled for naught."

Radak: "Bonav" are those who engaged in building it. This refers to Yerushalayim, the capitol of the kingship; the Bayis was built in it. The one who build the Bayis, i.e. his son Shlomo, with Hashem's approval, it is not for naught. In truth, it will be established.

Radak (5) #1: The Mizmor refers to Mashi'ach; Bayis is the Mikdash, and the city is Yerushalayim. After the Churban and exile of Yisrael, sometimes Yerushalayim was built, and sometimes destroyed. It was in the hands of Edom and Yishmaelim. They fought each other; these built it and these destroyed it, for Hashem did not desire their building or Shemirah.

Radak (5) #2: This refers to the endurance of any kingship - "u'Vayis Yivneh Lecha Hashem" (Divrei ha'Yamim I, 17:10). It will last only if Hashem wants.

Malbim: If Hashem's help will not accompany the building, the efforts will not help.

Rashi: The guard oversees it for no reason.

Radak: If Hashem will not guard Yerushalayim, the guards guard it for no reason. Rather, Hashem will guard it. He commanded that the Bnei Melech be in it. Since He desires it, He will guard the Bayis and the city. "Shakad" is engaging in and making efforts in a matter.

Radak (5) #1: Refer to 127:1:2:3

Radak (5) #2: If Hashem does not guard a city from enemies, guards will not help.

Malbim: Not only in carrying out a project, man's efforts will help only if there is help from above - rather, even what is already done, e.g. guarding a city that was built, if He does not guard it, man's efforts will not help at all.