I've had a number of commenters on here who have blamed Hezbollah's rise on Israel. Here's an article from the Times of London that explains what is and has been the driving factor behind Hezbollah. Iran's regional ambitions.
What are the links between Hezbollah and Iran? In 1982 Iran had almost no influence in Lebanon. The Lebanese Shi’ite bourgeoisie that had had close ties with Iran when it was ruled by the Shah was horrified by the advent of the clerics who created an Islamic republic.
Seeking a bridgehead in Lebanon, Iran asked its ambassador to Damascus, Ali Akbar Mohtashamipour, a radical mullah, to create one. Mohtashamipour decided to open a branch in Lebanon of the Iranian Hezbollah (the party of God).
After many meetings in Lebanon Mohtashamipour succeeded: in its founding statement it committed itself to the “creation of an Islamic republic in Lebanon”. To this end hundreds of Iranian mullahs, political “educators” and Islamic Revolutionary Guards were dispatched to Beirut.
Within two years several radical Shi’ite groups in Lebanon, including some with Marxist backgrounds, had united under the Hezbollah name and became the main force resisting the Israeli occupation of Lebanon after the expulsion of Yasser Arafat’s Palestine Liberation Organisation (PLO) in 1983.
Terror has been its principal weapon. Throughout the 1980s Hezbollah kidnapped more than 200 foreign nationals in Lebanon, most of them Americans or western Europeans (including Terry Waite, the Archbishop of Canterbury’s envoy). It organised the hijacking of civilian aircraft and more or less pioneered the idea of suicide bombings against American and French targets, killing almost 1,000 people, including 241 US marines in Beirut and 58 French paratroopers.
The campaign produced results. After Hezbollah’s attacks, France reduced its support for Saddam Hussein. America went further by supplying Iran with TOW anti-tank missiles, shipped via Israel, which helped to tip the Iran-Iraq war in favour of Iran. In exchange Iran ordered Hezbollah to release French and American hostages.
Once the Iran-Iraq war was over, Tehran found other uses for its Lebanese asset. It purged and then reshaped Hezbollah to influence the broader course of regional politics while using it to wage a low-intensity war against Israel.
In 2000, when the Israelis evacuated the strip they controlled in southern Lebanon, Tehran presented the event as the “first victory of Islam over the Zionist crusader camp” and Hezbollah was lauded across the Arab world. Hezbollah taunted the Israelis with billboards on the border reading, “If you return, we return”.
To prop up that myth, Tehran invested in a propaganda campaign that included television “documentaries”, feature films and books and magazine articles. The message was simple: while secular ideologies — from pan-Arabism to Arab socialism — had failed to liberate an inch of Arab territory, Islamism, in its Iranian Khomeinist version working through Hezbollah, had achieved “total victory” over Israel in Lebanon.
Since 1984 Iran has created branches of Hezbollah in more than 20 countries. None has equalled the success of the Lebanese branch, which until recently enjoyed something akin to cult status among Arabs, including non-Muslims, because of the way it stood up to Israel.
It's quite a long and detailed article and it explains much of what is going on in the Middle East today. It also points to why the Saudis and other Sunni-dominated states are very unhappy with Hezbollah. On the question of timing the article has this to say:
The pincer war launched by Hamas and Hezbollah against Israel is also related to domestic politics. In the occupied territories, Hamas needs to marginalise Mahmoud Abbas’s PLO and establish itself as the sole legitimate representative of the Palestinian people. In Lebanon, Hezbollah wants to prevent the consolidation of power in the hands of a new pro-American coalition government led by Fouad Siniora, the prime minister, and Walid Jumblatt, the Druze leader.
(Shi’ites make up about 40% of the population, Christians 39% and Sunnis, Druze and others the remainder.) If the pincer war against Israel is won, Iran would be able to expand its zone of influence, already taking shape in Iraq and assured in Syria, to take in Lebanon and Gaza. This would be the first time since the 7th century that Persian power has extended so far to the west.
The strategy is high risk. If the Israelis manage to crush Hamas and destroy Hezbollah’s military machine, Iran’s influence will diminish massively. Defeat could revive an internal Hezbollah debate between those who continue to support a total and exclusive alliance with Iran until the infidel, led by America, is driven out of the Middle East and those who want Hezbollah to distance itself from Tehran and emphasise its Lebanese identity. One reason why Hezbollah has found such little support among Arabs in Egypt and Saudi Arabia this time is the perception that it is fighting Israel on behalf of Iran, a Persian Shi’ite power that has been regarded by the majority of Arab Sunnis as an ancestral enemy.
There is much more at stake than just Lebanon. This may be one of the last chances to head off Iran without a major war. That this is understood by the West may be why the European calls for a ceasefire have been almost pro forma with no real urgency behind them.