You're right, this is interesting to think about.
With my husband, I knew right away, the moment I saw him that he was THE ONE, lol. The first date lasted three days, after which he went home to pack all his stuff and move in. We've been together 12 years. On paper, he was absolutely terrible, and if I had a list of things I wanted it would have been a dealbreaker. Now I always tell him he's "everything I never knew I always wanted". So that one was a gut instinct, instant, compatibility.
I've had others, too, where the realization that we could be friends or lovers grew over time. But it's always been instinctive, never a rational thing. I'm a very logical person, but "good on paper" never works for me. I have no lists at all these days, just waiting to see what wanders into my life.
The heartbreaking scenario for me always seems to be the one where you start out compatible - or at least THINKING you're compatible - and it slowly dissolves, becoming clearer and clearer that there isn't a future. I've gotten better at accepting that as I age, and can look at relationships as having a natural life span and energy of their own. In my youth I spent a lot of time trying to pump life back into dead partnerships, never with much success. As you said, enamoured or more likely needy.
It sounds like that might be where your friend is - pushing to keep something going that just isn't meant to be. I'm sorry to hear she's blaming herself, I have that tendency too, and it's not pretty.