Because God says that He will bless those who bless them. 
Genesis 12:3-“I will bless them that bless thee and curse them that curse thee, and in thee shall all the nations of the earth be blessed.” 

Isaiah 60:12-“For the nation that will not serve you (Israel) will perish; it will be utterly ruined.”

Because we owe them a debt for the blessings that we have received through them. 
Through Israel the promises and covenants were given. Through them came the law, the prophets and the Messiah who has opened up access to God for all. Also through the Jewish apostles, the gospel first went to the nations (Rom. 9:4-5; 15:27).

Because God’s gifts & call on them have not been revoked. 
Romans 11:28-“but as regards the election they are beloved for the sake of their forefathers. For the gifts and call of God are irrevocable.” 

Though most Jews have not yet responded to the gospel, this has not invalidated God’s plan for them, which He will ultimate fulfill. The covenant with Abraham has not been replaced with the New Covenant, nor has been set aside because of their failure to enter the New Covenant. Though the once-for-all sacrifice of Jesus has been fulfilled and replaced the Temple sacrifices, the eternal covenant God made with Abraham, Isaac, Jacob and their descendants has NOT been set aside.

Because God has promised to bring them back to their land. 
Amos 9:14-15-“’I will restore the fortunes of my people Israel, and they shall rebuild the ruined cities and inhabit them…I will plant them upon their land and they shall never again be plucked up out of the land which I have given them.’ Says the Lord your God.”

Ezekiel 37:21-22-“I will take the people of Israel from the nations among which they have gone, and will gather them from all sides and bring them to their own land, and I will make them one nation in the land, upon the mountains of Israel, and one king shall be king over them all. 

The re-gathering of Israel is prophesied in Ezekiel, Isaiah, Deuteronomy, Amos, Hosea, Jeremiah, by Jesus (Luke 21:24) and Paul in Romans 11 and is a key condition to the full flowering of the Messianic Age (Genesis 35:11).

Because the times of the Gentiles is almost fulfilled.
Luke 21:24-“Jerusalem will be trodden down underfoot by the Gentiles until the times of the Gentiles are fulfilled.” 
Jesus foresaw that the Roman Gentile armies would overthrow every vestige of Jewish rule in Jerusalem; but He also saw that the day would come when Gentile domination of Jerusalem would come to an end. In 70 AD, Jerusalem fell to the Roman army. In 1967, in the course of defending themselves from an attack from their Arab neighbors, the Israelis regained control of the entire city of Jerusalem for the first time since 70 AD, thus fulfilling the prophecy of Jesus (Luke 21:28).

Because we are to “comfort God’s people”. 
In Isaiah chapter 40 God speaks; “Comfort, comfort my people says you God. Speak tenderly to Jerusalem and cry to her that h er warfare is ended, that her iniquity is pardoned, that she has received from the Lord’s hand double for all her sins.” This is a call for Christians to comfort the Jewish people.

Isaiah also envisions that in their regathering, the leaders of the nations would help the Jewish people and nurture them: “Thus says the Lord God; ‘Behold I will lift up my hand to the nations, and raise my signal to the peoples; and they shall bring your sons in their bosom, and your daughters shall be carried on their shoulders. Kings shall be your foster brothers and their queens your nursing mothers” (Isaiah 49:22-23). God says that the Gentiles will nurture and assist the Jewish people in their reentry to their land. It is our divine mandate to help them in the fulfillment of their unique call. Gentile believers must do more than observe what God is doing with Jewish people. God is asking us to get involved in helping them.

Because God is working with them to work out His plan for world peace. 
Romans 11:15-“For if their rejection means the reconciliation of the world, what will their acceptance mean, but life form the dead.” The Messiah is coming back to Israel to be recognized by His own people, and He will reign from there with His overcoming saints from every tribe and tongue and people and nations. 

Isaiah 2:3-“For out of Zion shall go forth the law and the word of the Lord from Jerusalem. He shall judge between the nations, and shall decide for many peoples; and the shall beat their swords into plowshares and their spears into pruning hooks, nation shall not lift sword  against nation, neither shall they learn war any more.”  World peace is coming. The fullness of the Messianic Age is coming.