Love & Friendship.

I have heard lot of people started relationship as a friend ends up with love. So I would like to know the difference between these two. Is it acceptable being a friend for certain period and then become a lover afterwards?. Does it not amount to injuring friendship?. At the same time, one cannot love a person with out having friendship. Then, What would be the intention of the person who becomes a lover when he started his relationship as a good friend? Can I say that the person is cunning or wolf in sheep skin?. Because he hid his true motive. Whatever he did was to cheat the other person?. Explain please. I am confused.