Why is "pride" often considered the worst sin?

The sin of pride is especially insidious and perhaps ineradicable, and an absolute barrier to salvation: because it is the ultimate complacency that "I am good enough as I am".

Salvation is resurrection, and resurrection is a remaking such that we become wholly good, wholly motivated by love...

This includes our recognition that we need remaking, that we need to reject and leave-behind that of us which is dissonant with the euphony of divine creation.

But if we are spiritually-proud, we see no reason why we need to be remade to be fitted for Heaven. 

The proud Man wants, instead, that Heaven be fitted around himself as-is.

Such pride seems very common and normal, and is found among the despised, weak, poor and sick - as also (more obviously) among the strong, arrogant and famous.

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Note: Of course, pride is not the only blockage to salvation. Self hatred is another, because it is our-self who is resurrected. If we hate our self, we will not desire to be resurrected.