Thanks for the feedback. Here are three of my own experiences from recent years that illustrate the truth of that maxim.
1. I was in a relationship with one girl for a whole year. I fell hard for her and I would have done just about anything for her. But she was a raging narcissist and repeatedly f**ked with my head, emotions and self-esteem before she finally dumped me. I was devastated at the time but now I look back and realize I dodged a bullet, to say the least. It was a definite case of "what the f**k was I thinking?" She tried to contact me not long ago to try to patch things up but I ignored her and will continue to do so forever.
2. On the rebound from girl #1, I fell for an old acquaintance who had unexpectedly become flirty with me. When went on a couple of dates and had a good time, but I tried to move things along too fast and was rejected. I'm still friends with this girl and from my vantage point, I can see now what a trainwreck she is. I don't think I would have ended up happy with her.
3. Girl #3 is a stunner, an online friend I admired secretly from afar until she broke up with her boyfriend and then, not long after, suddenly took an interest in me. We went on several long dates and I saw a lot of potential for a relationship developing, but then she ceased all contact for reasons unknown (although I have my theories). I'm reasonably sure at this point that she was just using me to alleviate her boredom and now she's moved on to something else and no longer has a use for me. I was pretty upset because I really had feelings for her, but as the gauze of "pedestalization" falls away from my eyes, all of her flaws that I ignored earlier are coming into focus. A relationship with her would have been fun for a while but probably would've come to an ugly end due to our very different personalities. This happened recently enough that I'm still bummed about it, but I know that someday I'll be glad that it ended before I got in too deep.