Lamentations was originally part of the Book of Jeremiah. It was later isolated because it was read publicly at one of the feasts of Israel and included in the Five Megilloth (or scrolls; see introduction to the Book of Ruth). Lamentations is read each year at Tisha B'av, a fast commemorating the destruction of the temple in Jerusalem (2 Kgs. 25:8, 9) in both 586 B.C. and A.D. 70.  The style and content leave no doubt that Jeremiah, the "weeping prophet," is the author. The book focuses on the destruction of Jerusalem as observed by an eyewitness (cf Jer.37 -39) Jeremiah alternates between horrible accounts of the destruction of the city to the confessions of the people's deep sins, and then to his appeals to God for mercy.

Each of the five chapters of the book is a separate poem of mourning. The first four are acrostics (each of the 22 letters of the Hebr. alphabet are used to begin each stanza of the poem). The acrostic is a literary device that makes it easier to remember and abstractly suggests that the author has covered his subject from beginning to end.

It may also be that Jeremiah, under the inspiration of the Holy Spirit, made himself conform to this method to keep his grief from overwhelming him. Lamentations is a valuable commentary on sin and the consequences, and about the return to God for mercy (Lam. 3:22 -24). [Source for Introduction of chapter: Hebrew Greek Key Study Bible KJV edited by Spiros Zodhiates, Th.D. AMG Publishers, Chattanooga, TN 37422]

My Few Notes:  In summary, God said to the people via Moses, if they transgressed His laws they will get curse rather than the blessings if they followed.  the people did not follow Him despite saying is Exodus 24:7 that they would.   Hence the curse inclusive of being sent into captivity for seventy years came upon them.  [see Jer 25:11 And this whole land shall be a desolation, and an astonishment; and these nations shall serve the king of Babylon seventy years. Jer 25:12 And it shall come to pass, when seventy years are accomplished, that I will punish the king of Babylon, and that nation, saith the LORD, for their iniquity, and the land of the Chaldeans, and will make it perpetual desolations. Jer 29:10 For thus saith the LORD, That after seventy years be accomplished at Babylon I will visit you, and perform my good word toward you, in causing you to return to this place. Dan 9:2 In the first year of his reign I Daniel understood by books the number of the years, whereof the word of the LORD came to Jeremiah the prophet, that he would accomplish seventy years in the desolations of Jerusalem].

Daniel and others were taken into Babylon by a conquering king.  At the end of the seventy years Daniel made supplication as mentioned by Solomon in 2Chron 7:14 and because he was much loved and obedient to god, god showed him what will happen to his people, Israel, which included the return of the people to Jerusalem. This happened under Ezra and Nehimiah.  [2Ch 7:14 If my people, which are called by my name, shall humble themselves, and pray, and seek my face, and turn from their wicked ways; then will I hear from heaven, and will forgive their sin, and will heal their land].

chapter 1 verse 7.
1:7 “Mocked at her Sabbaths” because keeping them did not save Israeel who sinned in many other ways.
1:9 O LORD, behold my affliction: for the enemy hath magnified himself. Lam 1:10 The adversary hath spread out his hand upon all her pleasant things: for she hath seen that the heathen entered into her sanctuary, whom thou didst command that they should not enter into thy congregation. Dan 8:11 Yea, he magnified himself even to the prince of the host, and by him the daily sacrifice was taken away, and the place of his sanctuary was cast down. Anchiotus Epithany did this by entering and polluting the sanctuary as narrated in book of Maccabees.

2:6 “the LORD hath caused the solemn feasts and sabbaths to be forgotten in Zion, and hath despised in the indignation of his anger the king and the priest. Lam 2:7 The Lord hath cast off his altar, he hath abhorred his sanctuary, he hath given up into the hand of the enemy the walls of her palaces; they have made a noise in the house of the LORD, as in the day of a solemn feast.” As in Eze and other places the Israelites used God’s house for pagan worship as adopted from heathen. As a result they were punished (see 4:10).
Lam 3:22 It is of the LORD'S mercies that we are not consumed, because his compassion fail not.
Lam 3:23 They are new every morning: great is thy faithfulness.
Lam 3:24 The LORD is my portion, saith my soul; therefore will I hope in him.
Lam 3:25 The LORD is good unto them that wait for him, to the soul that seeketh him.
Lam 3:33 For he doth not afflict willingly nor grieve the children of men.
Lam 4:10 The hands of the pitiful women have sodden their own children: they were their meat in the destruction of the daughter of my people. Lam 4:11 The LORD hath accomplished his fury; he hath poured out his fierce anger, and hath kindled a fire in Zion, and it hath devoured the foundations thereof.

Lam 5 Plea for mercy.
Associated reading: Dan 9:2 In the first year of his reign I Daniel understood by books the number of the years, whereof the word of the LORD came to Jeremiah the prophet, that he would accomplish seventy years in the desolations of Jerusalem. (The reason for Lamentations)
Dan 9:3 And I set my face unto the Lord God, to seek by prayer and supplications, with fasting, and sackcloth, and ashes: Dan 9:4 And I prayed unto the LORD my God, and made my confession, and said, O Lord, the great and dreadful God, keeping the covenant and mercy to them that love him, and to them that keep his commandments; Dan 9:5 We have sinned, and have committed iniquity, and have done wickedly, and have rebelled, even by departing from thy precepts and from thy judgements: Dan 9:6 Neither have we hearkened unto thy servants the prophets, which spake in thy name to our kings, our princes, and our fathers, and to all the people of the land. … to Dan 9:10 Neither have we obeyed the voice of the LORD our God, to walk in his laws, which he set before us by his servants the prophets. Dan 9:11 Yea, all Israel have transgressed thy law, even by departing, that they might not obey thy voice; therefore the curse is poured upon us, and the oath that is written in the law of Moses the servant of God, because we have sinned against him. Dan 9:12 And he hath confirmed his words, which he spake against us, and against our judges that judged us, by bringing upon us a great evil: for under the whole heaven hath not been done as hath been done upon Jerusalem. Dan 9:13 As it is written in the law of Moses, all this evil is come upon us: yet made we not our prayer before the LORD our God, that we might turn from our iniquities, and understand thy truth. All of Daniel’s pray to v20