paragraph:  Shelley was, in the main, not only an exquisite but a trustworthy critic; and no man was more absolutely above being influenced by the fanfaronade of rank or the din of popularity.  These criticisms are therefore not to be lightly set aside, nor are they unintelligible.  Perhaps those admirers of the clearer and more consistent nature, who exalt him to the rank of a greater poet, are misled by the amiable love of one of the purest characters in the history of our literature.  There is at least no difficulty in understanding why he should have been, as it were, concussed by Byron's greater massiveness and energy into a sense--easy to an impassioned devotee--of inferiority.  Similarly, most of the estimates-- many already reversed, others reversible--by the men of that age, of each other, can be explained.  We can see how it was that Shelley overestimated both the character and the powers of Hunt; and Byron depreciated Keats, and was ultimately repelled by Wordsworth, and held out his hand to meet the manly grasp of Scott.  The one enigma of their criticism is the respect that they joined in paying to the witty, genial, shallow, worldly, musical Tom Moore. 
question: What character trait of Byron's was to influence Shelley's usually trustworthy critiques of fellow writers?
answer: Superiority