Friendship is something precious. It is a relationship between two person who find themselves inseparable with the other. You can have a lot of friends all in the same time. In fact the more friends you have the better. Real friendship is a beautiful bonding that could last all hardship life can bring.
"What is a friend? A single soul dwelling in two bodies" Aristotle
Lovers on the other hand is different in such a way that the relationship are expected to be bound in marriage. Compared to friendship, you can only have one lover. One that you will be committed to whatever life may bring. There are a lot of difficulties lovers will eventually encounter, one is distance... loooooong distance. Thus a lot of effort is necessary to make it last a lifetime.
Friends turn lovers, sweet right? Yeah, but only if it will do last a lifetime. Because if you fail being lovers, it is too difficult, sometimes impossible, to make yourselves friends again.