Opposites attract: The most common reason behind this question is the gender. Both can't stop the affection towards each other at a point of time. Somehow the feeling of having a friend of opposite gender mentally creates a different world of its own!
Lust: The physical love kills the friendship between a girl and a boy. They can't be friends for a long time especially if both are single or at least one of them is single. The physical attraction is considered as the most common problem in friendship.
Love: A girl and boy can never be friends if the feeling of love rises in the heart. Often friends become lifetime partners because the compatibility is best and through friendship, they know each other better. But sometimes, love cannot be from both the sides. It is not necessary for both to think and feel the same for each other.
Nature: A flirtatious nature can spoil friendship to a greater extent. This is why a girl and a boy can never be friends. Flirty nature of the boy can irritate the girl at times especially if he flirts with her!
These are the few reasons as to why a boy and girl can never stay as friends. Different nature and attitude can make them severe all ties with each other and move on!