What causes the Prophets to be infallible?

The sources and foundations of 'isma (Infallibility) can be summarized in the two following principles:
1. The Messengers (and certain saints) possess such a profound degree of gnosis, such a subtle mode of awareness of God, that they would not exchange their contentment with God for anything else. In other words, their perception of the divine grandeur, of His bounty and majesty, is such that they cannot see anything apart from His reality, and they can entertain no thought apart from that of seeking the satisfaction of God: This station of gnosis is referred to in the following saying of Imam 'Ali (as):

'I have seen nothing without seeing God before it, after it, and along with it.'

And Imam Sadiq (as) said:

'I worship God out of love for Him; and this is the worship of the great ones. ' [1]

2. The perfect awareness, on the part of the Messengers, of the beatific consequences of obedience, and the terrible retribution that follows disobedience, gives rise to their immunity from disobedience to God. Of course, 'isma in all its fullness is the exclusive preserve of the special saints of God, but in fact some pious believers are immune from the commission of sins in many spheres of their activity. For instance, a pious man would never commit suicide, at any price, nor would he kill innocent people.

The perfect awareness, on the part of the Messengers, of the beatific consequences of obedience, and the terrible retribution that follows disobedience, gives rise to their immunity from disobedience to God. Of course, 'isma in all its fullness is the exclusive preserve of the special saints of God, but in fact some pious believers are immune from the commission of sins in many spheres of their activity. For instance, a pious man would never commit suicide, at any price, nor would he kill innocent people.

Imam 'Ali (as) says, as regards such people: 

'In relation to Heaven, such people are as those who have beheld it, and benefit from its graces; in relation to Hell, they are as those who have beheld it, and are undergoing its punishment.' [2]

There are also ordinary people who benefit from a kind of 'immunity' in some of their affairs. For instance, nobody would, at whatever price, touch a naked, live electrical wire. It is clear that immunity in such instances arises out of the Individual's certain knowledge of the negative consequences of a given action: were such certain knowledge of the dangerous consequences of sin to be attained, it would be a potent source of rendering a person immune from sin.

1. al-Majlisi, Bihar al-anwar (Beirut, 1403/ 1982) vol. 70, p. 2 2.
2. Nahj al-balagha, Sermon no. 193.