Why do most Christians love to share their faith but put up a wall when anyone wants to do the same to them?

Example : My born again friend always seeks to witness of Christ and yet when someone of another Christian Faith tries to share their love of Christ they put up a wall and will not even listen and weigh what is being said? If we truly love christ and recognize we are imperfect, should we not be open to new truth and understanding from God. Moses took advice from his father n Law, Peter listened to Paul at times, Even if we have a "holier than thou" attitude shouldn't we still test all things and seek after that which is good?