I guess you are questioning why Catholics believe that the pope is infallible when teaching issues of Faith and Morals even though he is a human being who can sin or be fallible…It is the same reason why Jesus said this in Matt 23
Matt 23:1-12: “Then Jesus spoke to the multitudes and to His disciples, saying: "The scribes and the Pharisees sit in Moses’ seat. "Therefore whatever they tell you to observe, that observe and do, but do not do according to their works; for they say, and do not do.
Note that Jesus didnt think much about the Pharisees but still he knew that when they teach the law from the seat of Moses everyone has and will to submit to the teaching. It is imperative.
How much more would it mean to be teaching from the Seat of Peter? Here is what Jesus had to say to Peter in Luke 22
31“Simon, Simon, Satan has asked to sift you(Note that ‘you’ in Greek is plural referring to the apostles) as wheat. 32But I have prayed for you, Simon, that your faith may not fail. And when you have turned back, strengthen your brothers.”
Jesus not only hands the keys to Peter but he says that his faith will not fail…So when the pope who occupies the seat of Peter is teaching issues of faith or morals, he is speaking using God’s authority and not his own…There is a difference between the Pope having his opinion about the center of the solar system and teaching issues of Faith and Morals…The pope while teaching issues or morals and faith represents the teaching of the church that Christ instituted and is guided by the holy spirit to be infallible. There is numerous mentions about the holy spirit protecting the truth and the teachings.
Read this: catholic.com/library/Papal_Infallibility.asp