Blossomed into love
It is common for heterosexual friendship never to stay in one steady state it usually develops into something. It is common that when man and woman call each other friends and spend much time together they get to know each other better and so they are close enough. When they are close enough it is hard to remain ignorant.
Actually, in most cases woman are able to be friends and they are thirsty for nothing more than friendship. Though, men are people of different vision of the world and there is no way for man to remain ignorant towards attractive women. So, sometimes friendship never lasts for a long and usually blossoms into love between people.
It is great when both of former friends are in love and they want to go on together, but it is more common that one is in love and the other one still believes that they are just friends. In this case there is no friendship anymore and will be no love between them. So it is better to walk away as such relationships is a way to nowhere.
But, as for those who fall in love with each other, their friendship is such a kind of friendship we call the blossomed into love friendship. People say that it is great when a couple has friendly ground of their relations. Though, there are also those who say that there is no place for passion and love where there is a moderate feeling of friendship.