It's rather hard to state definitively; as Richard Overy puts it in The Bombing War: Europe 1939-1945, "for the historian... any attempt to suggest what might have led to social breakdown even in one city is an exercise in speculation." Certainly before the war there was a rather apocalyptic view of strategic bombing, theorists like Giulio Douhet suggesting that an enemy could be defeated by shattering civilian will to resist, so thought was given to civil defence; Stanley Baldwin gave a speech in 1932, ["A Fear for the Future"] (http://en.wikisource.org/wiki/A_Fear_For_The_Future) (including the famous line "the bomber will always get through"), in which he said "I will not pretend that we are not taking our precautions in this country". As tension built in the run-up to war organisations were put in place e.g. through the Air Raid Precautions act of 1937 and with the establishment of the Woman's Voluntary Service (WVS) in 1938, so the population were preparing for bombing raids (particularly the prospect of gas attack), efforts increasing with the declaration of war in 1939 including the evacuation of almost 1.5 million people from threatened cities (though with no actual attacks, almost two thirds had returned by January 1940).
Once The Blitz did begin in earnest, the damage inflicted was not as severe as pre-war worst case scenarios; the Luftwaffe was not a strategic air force, though as the largest in the world at the time it still presented a major threat. The switch to night bombing increased the pressure on the defensive infrastructure, with shelters that were limited at the best of times not being intended for habitation, lacking bedding, sanitary facilities etc., but the government did respond, led by newly appointed Home Secretary Herbert Morrison, though shelter construction competed for resources with anti-invasion defences. In some areas large numbers of people became "trekkers" who left cities at night to sleep in nearby towns, villages, or even open country and tents; though Coventry suffered a devastating raid on 14/15 November 1940, there were comparatively few casualties.
The image of a stoic population saying "Britain can take it!", buoyed by government efforts such as the documentary of the same name, was undoubtedly not universal, debunked to varying degrees by works such as The Myth of the Blitz, but Overy concludes that the key elements in maintaining stability were material assistance from state and local authorities in the form of assistance, food and shelter after attacks, and various psychological coping mechanisms including religion and fatalistic bravado allowing people to normalize the experience and continue their lives.
I think as Bigglesworth_ wrote, the answer lies in the prior overestimation of the effectiveness of strategic bombing in the lead up to the war.
The First World War had driven great technological developments in warfare, the Tank, Submarine and the aircraft (also aircraft carrier) had both proven to be effective tools in warfare. Development and design during the war, and subsequent further advances led commentators and theorists on all sides to expect 'great' things from bombing campaigns.
For the British, bombing had been proven to be effective, in 1920 British forces returned to Somaliland to fight the Dervish forces of the 'Mad Mullah' Mohammed Abdullah Hassan, who had waged a 20 year campaign of resistance in the Horn of Africa, against prospective Italian, British and Ethiopian conquerors. The RAF was deployed to help destroy the Dervish state. Using coordinated air support tactics, the RAF was able to systematically destroy Hassan's defences, level his capital, and with the assistance of ground forces, occupy his territory, the campaign took a month, and with a cost of around £77k it was dubbed 'the cheapest war in History'. The next year the RAF replicated their success on a larger scale in the suppression of a large revolt in Iraq.
The effectiveness of strategic bombing was confirmed in the minds of those in charge. Hugh Trenchard, Chief of the Air Staff and instrumental in the foundation and maintenance of the peacetime RAF perceived great things in the future of strategic bombing, and pointed to its effectiveness in maintaining control within the Empire.
In the inter-war period official government projections took stock of the German raids that began in 1915 with Zeppelins, and then continued throughout the war with Gothas and Giant heavy bombers. The raids killed around 1500 civilians during the war, injuring over twice that number. From this, it was estimated that each ton of bombs dropped amounted to about 50 dead. in 37, the defence committee was informed that Germany would be able to launch a continuous air assault on Britain lasting sixty days. Estimated casualties were around 600,000, injuries over a million. The attacks in the First World War also had induced panic and hysteria dubbed 'Zeppelinitis', officials expected similar panic in any future bombing campaigns. Inter-war experts and politicians utilised the same sort of terms and theories that post war planners used about the potential effects of atomic bombing. Indeed, Baldwin's 'Fear for the Future' speech was couched in the terms of disarmament, and the British attitude to German rearmament beginning in '33 was to attempt to out produce Germany as discouragement - just like the MAD doctrine.
The hyperbole of bombings psychological effect coupled with the over estimation of German capabilities, had the effect of producing a country that was well prepared to weather an extensive bombing campaign. Organisation on the ground: with volunteer fire services, air wardens, extensive air-raid shelter building and the evacuation combined with Advances in air defences, fighter technology and Radar, meant that the effects of the bombing was muted. The impact on morale that strategic bombing was supposed to have had never truly been measured, and had indeed been massively overstated prior to the war. Despite the evidence from the Spanish Civil War, where the bombings of Spanish cities were generally observed to firm up resistance, uniting disparate elements in a shared trauma, and focussing their aggression against their attackers.
I think its also important to note that the 'Blitz Spirit' was not confined to Britain. The RAFs bombing campaign in Germany, including the razing of Cologne never caused the absolute collapse of morale that was hoped for.
If you want a more narrative account of Bomber Commands operation during the war, I'd recommend Patrick Bishop's Bomber Boys. The initial chapter is pretty pertinent to this.