Islam has a moral concept called sacred hate and sacred love, in Arabic, it is al wala wal bara. What does Allah love? Muslims. What does Allah hate? Kafirs. There are 13 verses in the Koran that say that a Muslim can never be a true friend to Kafirs. They can be friendly with Kafirs, but never a true friend.
It is sacred doctrine that the Kafir can be hated, kidnapped, tortured, deceived and enslaved. There are no good words about the Kafir in the Koran.
This hating-for-Allah directed to all Kafirs poisons our relations with Muslims. It plants a seed of doubt in friendship with them. Sadly, true friendship with a Kafir means not being obedient to Allah and Islam.
This talk is based on a chapter in my new book Measuring Mohammed.
See also my older article on the topic.