When you first start to date someone, they are neither a friend nor a lover. But, as time goes on, they will need to become both.
Sometimes it is first a friend and then a lover. And other times it is a lover and then a friend. Regardless of the order, both need to happen for a successful relationship.
If you are only lovers and not friends, then the relationship is merely one of carnal pleasure and self gratification. Likewise, if you are “buds” but nothing more, then your relationship will never develop into something more meaningful and intimate.
In both of these scenarios, time is of the essence. The longer that you spend as only lovers, the greater the likelihood that you will never become friends. After all, as time goes on, you learn to only think of the other person in terms of your sexual needs.
Likewise, the longer you spend as friends, the greater the likelihood that you will never become lovers. At a certain point in time, you will stop thinking of that person as a potential lover and only think of them as a friend. When that happens, the thought of making love to them will just seem, well, perverted.
In short, relationships have to keep moving forward. If they don’t, they will deteriorate.