Seven proximate causes compelled Israel to take pre-emptive defensive military action on June 6, 1967; five of them constituted direct causi belli, while the two others were of a different nature: one – political and the other being water
Egyptian blockade against Israeli shipping in international waters - Straits of Tiran - and the failure of the maritime nations to honour their undertakings given to Israel following the Suez Campaign, to challenge that blockade, if imposed by Egypt;
UN acquiescence in removal of the peacekeeping force from
In 1956, with the threat of Soviet interference hanging over their heads if Israel failed to withdraw from the Suez Canal, Israel, Britain, and France had conducted intensive diplomatic negotiations with the US and the UN. These produced two “good faith” agreements which gave Israel some degree of security in return for her withdrawal from the Canal zone:
The first, between Nasser and the Secretary General, whereby Egypt was promised that it would have the right to demand the removal of UN forces whether the peace keepers had completed their mission. from its territory, but only after the General Assembly had considered the matteer;
The second agreement made between the US Secretary of State and Israel’s Foreign Minister, whereby the US undertook that it would regard any Egyptian attempt to interfere with Israel’s right of free passage through the Straits of Tiran or any fedayeen attacks emanating from Gaza as causi belli.
It had been agreed that the occurrence of such events would entitle Israel to act in self defence under Article 51 of the UN Charter, and she undertook to inform the United States of any such intention. In the event the UN were to abandon any of its responsibility in Egypt, the United States, France and Britain agreed to support Israel if it found itself in the position of having to take action in self defence against Egyptian aggression. The efficacy of this arrangement was seen in the fact that between March 1957 and May 1967, not a single episode of armed attack took place against Israel from the Gaza strip. , , 
Massing of troops on Egyptian, Syrian and Lebanese borders poised for invasion; and
The Jordanian attack on that part of Jerusalem held by Israel since 1948, and on the UN enclave around Government House despite Israeli approaches to Jordan that if she remained uninvolved, Israel would not attack her.  p.184
The onetime Commander of the UN Observer Forces in the area, General Odd Bull, notes that the roots of the 1967 conflict started much earlier in 1964 (see Odd Bull, War and peace in the Middle East: The Experiences and Views of a UN Observer, Leo Cooper; London, 1976, pp. 72-78) On May 28, Israel started to pump water from the River Jordan to irrigate the Negev- the desert southern part of Israel. The quantity to be taken was within that allocated to Israel in Eric Johnson's 1955 plan for sharing the combined water of the Jordan River and its tributaries between Israel and its neighbours. 
The Arab governments at a meeting on September 7, 1964, objected to the development of the Negev in this manner and resolved to counter Israel's action by drawing off water from two of the three tributaries to the Jordan (Hasbani in Lebanon and Baniyas in Syria), diverting them eastward and then southwards into the River Yarmuk within Jordanian territory. Israel reacted and notified the Armistice Commission and the UN Security Council that it would view the implementation of such plans as aggression and a breach of the Armistice Agreements. (Israeli Notes to the Security Council following th don’t do anything second Arab summit conference, S/5980, 18 September, and S/6020, 19-October 1964  When the Syrian government, inside its own borders, actually attempted to divert the Banyas, Israel responded by three army and air-force attacks on the site of the diversion. 
In passing, it is worth pointing out that Odd Bull’s observation that the conflict started in 1964 is misleading. In fact it started much earlier when Britain agreed to transfer its control over the headwaters of the Jordan to France of the under Franco-British [Boundary] Convention signed on December 23, 1920.
Terrorist Activity Emanating from Jordan
Terrorist infiltrations from Jordan and their attacks on civilian settlements inside Israel contributed to Israel’s insecurity. On 1 April 1967, Palestinian infiltrators blew up a water pump at a kibbutz on the Lebanese border. Later on April 7, 1967 the Syrians fired on two Israeli tractors entering the Demilitarised Zone located between itself and Israel. The IDF fired back. The battle on land then escalated into one in the air when Israel planes attacked Syrian installations and tangled up with the Syrian Air Force. A huge dogfight took place over Damascus involving an estimated 130 aircraft in which four Syrian MiGs were shot down and Israeli Mirages did a victory loop around the city to rub it in.
Soviet disinformation tactics play a crucial role in instigating war. The Israeli air attacks over Syria put the Soviets in the position of being able to feed Syria and ultimately Egypt with disinformation about Israel’s supposed intentions such that they provoked Egypt into taking military action against Israel. Commencing on May 8, 1966, a TASS cable from Damascus made the first mention of a suspicious concentration and movement of Israeli troops sighted lately on the border with Syria. By May 21 it was being asserted that about a third of the Israeli army was being transferred to the Syrian border. (see Isabella Ginor, The Cold War's Longest Cover Up: How and Why The USSR Instigated The 1967 War, vol. 7, Issue #3,Meria Journal, Sept. 2003) The Russian Foreign Ministry sent at least eight warning notes to Israel's diplomats, alleging Israeli troop build-up on the Syrian border, none of which was true. At one point Israeli Premier Eshkol suggested that Soviet Ambassador Sergei Chuvakhin go to the northern border and check for himself. Chuvakhin replied that his job was to communicate Soviet truths and not to test them.
“The repetition of these charges, together with increasingly acrimonious Soviet statements and the encouragement of Syria to undertake actions (which indeed provoked a forceful Israeli response climaxing on April 7, 1967) were part of a deliberate escalation designed to prepare the ground for harnessing Egypt to the end is in the military confrontation being prepared and to draw an Israeli strike against Egypt as well.” 
General Odd Bull commented that in April 1967 during the period when Russian was issuing its warnings concerning Israeli troop build up, the Israelis had agreed to a UN inspection of the Demilitarized Zones, a somewhat strange thing to do if the allegations had been true.
“The Soviet Union warned Syria about Israeli troop concentrations and the likelihood of attack. Nasser gave credence to these warnings and resolved to support Syria by concentrating Egyptian forces in Sinai….Nasser was obliged to act if his reputation in the Arab world was not to suffer because he had been subjected to a lot of criticism on the ground that he was sheltering behind UNEF. Presumably his hope was that his gestures of support for Syria would be sufficient to dissuade the Israelis from attacking Syria.”  p. 105
 Michael K. Carroll, From Peace Keeping To War:The United Nations and the Withdrawal of UNEF, 9 Meria Volume 9, No. 2, - June 2005
 Michael Oren, What Are the Origins of the Middle East Crisis? Six Days of War
 Branislav L. Slantchev National Security Strategy:The Arab-Israeli Conﬂict, 1916-1978 (pdf file) )
 Masahiro Murakami, Managing Water for Peace in the Middle East: Alternative Strategies, United Nations University Press, New York, 1995
 Israeli Notes to the Security Council following the second Arab summit conference, S/5980, 18 September, and S/6020, 19 October 1964.
 Lilach Grunfeld, Jordan River Dispute, ICE Case Studies, Case Number: 6, Spring 1997
 Isabella Ginor, The Cold War's Longest Cover Up: How and Why The USSR Instigated The 1967 War, vol. 7, Issue #3,Meria Journal, Sept. 2003) PDF file
 Greg Goebel Notes From Six Days of War, Chapter 2 of 4 / 28 Aug 05