Over the years, I have observed that we have different types of friends. These friendship types seem to form a hierarchy somewhat like the one described by Maslow. Ideally, and I cannot overemphasize this, friendships progress up the hierarchy.

At the bottom are mere Acquaintances. In the context of a singles club, these are people you have met. You may have sat next to each other at a dinner or a happy hour and had a conversation. In the context of the world at large, this group may include co-workers, clerks in stores you frequent, and so forth.
At the next level are Friends. It is difficult to define precisely when an Acquaintance becomes a Friend, but at some point this happens. These are the people you seek out at events, people you want to speak with to see what is new with them, and people who share a common interest with you. You might have attended an special event with these people, or met for tennis or a game of golf. In the case of the opposite gender, these are people you may have dated. However, it is important to note that you will develop friendships with members of both sexes, and indeed, you should develop friendships with members of both sexes.
As we become comfortable with some of our Friends, we develop an intimate friendship with them. An Intimate Friend is someone you can share your deep dark secrets with, and not worry about them running around telling the world about them. These are people you can confide in, and they are very important in the scheme of things. In general, we have very few Intimate Friends, in many cases, we may only have one. Again, gender differences are not significant, and ideally we might have at one Intimate Friend of each gender.
At the top of the hierarchy is our Lover. Notice that I do not use the plural at this level. In this ideal world I keep referring to, we only have one Lover, one person we are truly devoted to. This Lover might become our spouse, but I would not create a higher level in my hierarchy for a spouse. If we have multiple Lovers, we are probably working our way down the hierarchy rather than up it.
And therein lies the key to this whole hierarchy. In an ideal situation, we work up the pyramid. We make Acquaintances, some of whom become our Friends. As our friendships mature and grow, some of those become our Intimate Friends. And finally, one special Intimate Friend becomes our Lover.
The problem is that we often do not have the patience to go up the pyramid. We are anxious to find a Lover, so we take a few shortcuts and go directly from Acquaintance to Lover. Because we have skipped the Friend level, we may not have a whole lot in common, other than a desire to go to bed with each other. And because we have skipped the Intimate Friend level, we may not be able to talk with each other and work out any problems that arise.
Our situation changes throughout life. When we are newly single, especially if we are recently divorced, we are often a bit desperate (a harsh word, but accurate). We are suddenly without something, a Lover, and we are very anxious to get that back. As we become more comfortably single, we can relax and take our time working up the pyramid. And there are other events in our lives that can put us into this shortcut mode.
However, if we work our way up to a Lover, that Lover is also a Friend and an Intimate Friend. We can talk with them about anything that is troubling us, because they are an Intimate Friend. We can have fun together because they are our Friend. And that is really wonderful.