Depends on the people involved....I knew someone who was very good friends with a guy back in high school. They lost touch, got reacquainted two decades later, and are now very happily married.
My wife's last boyfriend was previously her best friend. It was someone she had a very high comfort level with. Eventually, she grew tired of his shiftlessness, his drunkenness, and his other shortcomings (OMG, a Freudian!!!!!! 😛), and the engagement ended.
About four months into our relationship, I spent Saturday morning at my then girlfriend's condo. Just after finishing lunch (and before heading off to work), the phone rang. Surprise! It was her last boyfriend. "How did you get my number?" "I found it on the internet." Some small talk. "So," he asked, "like, what have you been up to?" "Well, what have I been up to? Would you like to speak to him? He's right here."
Sometimes you go fishing and don't catch anything...and then, there are times when the pole malfunctions.
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Here's one: Before I met the Miz, I had tried a fling with a woman at my job, after my last relationship ended. I was in a relationship with her behind my then girlfriend's back, no excuses, a stupid thing to do, really. The woman really, really liked me. However, she wasn't good relationship material. After I met the Miz, I took stock of the woman's positives, and made her a good friend of OURS...we'd go places, do things. In the long run, it didn't work out. The Miz thought she was nuts, and having known her for quite a while, I was willing to concede that. However, she had a great sense of humor, and I hated losing that...
Like I said, it depends on the participants involved.