Sometimes the reason for an error in faith or teaching is due to mixing with different dogmas, or being influenced by them and their teachers, or by being disciples of such teachers or their writings. Sometimes the reason for an error in faith is due to one's sticking to one's own opinion, neither accepting any change nor obeying the Church. Most probably the reason behind this is pride in the heart convincing the person that he is right and whoever objects to his opinion is wrong, and that he understands what no-one else does.