Why do people say that Israel had no right to invade the West Bank & Gaza Strip in 1967...?

The 1967 Six-Day War involved three distinct battlefronts, tied together by a shared desire on the part of the surrounding Arab states to eliminate Israel and erase the shame of their defeat 19 years earlier when they failed to destroy the nascent Jewish state.

Egypt, the largest Arab state with a population of 31 million, massed troops on its border with Israel and imposed a naval blockade of Israel’s southern port, an act of war. Confronted with these aggressive moves, and the Arab leaders' promises to destroy the Jewish state, Israel launched a pre-emptive strike against the Egyptian army and airforce. Egypt’s air force was quickly crippled, and a well-executed Israeli ground offensive routed the Egyptian forces in Gaza and the Sinai peninsula in four days.

Buoyed by false reports of Egyptian success, Jordan initiated offensive actions against Israel from the eastern portion of Jerusalem and from lands it occupied west of the Jordan river (the West Bank). Israeli forces responded by attacking Jordanian military positions. After a three days of fierce fighting, especially in and around Jerusalem, Israeli forces defeated the Jordanians and gained control of all of Jerusalem as well as the West Bank, the historical heartland of the Jewish people known to Israelis as Judea and Samaria.

Following an air attack by the Syrians on the first day of the war, Israel dealt a shattering blow to the Syrian air force. Hostilites continued in the days that followed, and on fifth day of the war, the Israelis mustered enough forces to remove the Syrian threat from the Golan Heights. This difficult operation was completed the following day, bringing the active phase of the war to a close.

Churchill White paper:
"The terms of the Declaration referred to do not contemplate that Palestine as a whole should be converted into a Jewish National Home, but that such a Home should be founded "in Palestine." "

suggesting Balfour intended some of what was then Palestine would be for the Palestinian Arabs who have lived in that area since prehistoric times:
"part, or perhaps the majority, of the Moslem Arabs in this country descended from local inhabitants, mainly Christians and Jews, who had converted after the Islamic conquest in the seventh century AD (Shahan 1971; Mc Graw Donner 1981).
These local inhabitants, in turn, were descendants of the core population that had lived in the area for several centuries, some even since prehistorical times (Gil 1992)."

Yes, and Israel withdrew from Gaza and look what's happened to it. Those "poor Palestinians" can't even keep control in their own house.

If all the other people in the world agreed with you, Sept 11th would never have happened.