Perhaps he resolved his doubts, an additional cause being added, for sorrow seized upon him and his countenance fell. Therefore, he took the sorrow which he felt as an indication that he had been sacrificing what was not pleasing or approved of, when joy and happiness would have been suited to one who was sacrificing with purity of heart and spirit.
Why is it that the expression used is not, because you do not offer rightly; but, because (or unless) you do not divide rightly? (Genesis 4:7).