My MIL has many wonderful things about her (kind, generous, puts people at ease, an incredible hostess to make anyone comfortable who stops by to stay for any amount of time it seems - I sometimes wish I had that gift), and of course, a few things that drive me mad, like any family. But she has had a terrible track record with romance. It’s heart-breaking. I don’t want to list it all as outing, but my husband’s father died when he was a baby in a terrible accident and she has built up an epic love story around him, and all her relationships since then have ended in disaster. I’ve noticed three things:
1. She is the queen of viewing through rose-tinted glasses. The thing is, when you look at the world through rose-tinted glasses, all the red flags just look like regular flags. My husband’s father has been dead over 30 years now. It sounds like he was a fun bloke, life of the party, laidback,and easy-going. But he was also unfaithful to all his previous partners and started a relationship with MIL by being unfaithful to someone else. I do wonder, if he’d lived, if they’d have actually stayed married all this time. But since he didn’t, she can say it was a perfect love and that she lost her soulmate. Sigh.
2. She seems to think logic and love don’t belong together. She will repeat the same self-destructive behaviors again and again in the name of love. And this is a woman who is absolutely brilliant in other areas of life. But it’s as if she’s afraid if she sits down and thinks things through (ie, “X is doing something cruel, someone who loves you would not be cruel, therefore X does not love me”) coolly and logically as she would in other areas of her life, it’s not in the “spirit” of loving someone. 
I mean, I think if the way someone treats you can’t stand up to that kind of basic logic, we’re not talking about love or souls, but probably just another utter bastard.
3. And this is maybe the MOST important in my opinion: she refuses to get any counseling/talk to someone, etc. Sometimes relationships just end, yes, but if every one of your relationships explodes spectacularly and always leaves you in deep grief, you NEED some support... and you need to learn how to have healthier relationships. It’s a bit like going to school for something most of us never learned (how to have healthy, long-term adult relationships), unless we just got lucky and had it modeled for us by parents, etc. So MIL goes into the next relationship, bruised in spirit, with no more relationship skills (like I said, she’s brilliant in other areas) nor understanding than she had before. It’s so tough on her and difficult to watch. And you can have a conversation about it with her, she can agree with everything (that she should be treated more kindly, that no one should do X or Y to her), and then let some arsehole she hopes is her soulmate does X and Y again and again...
Of course, the awkward bit is that DH and I seem to have this kind of “soulmate”-y marriage to outsiders. We don’t really. There’s no magic potion. What it really is, is a commitment: to communicate and not to leave. We both had parents who would use avoidance and emotional blackmail, so we agreed at the beginning we wouldn’t have a marriage where someone ever storms out. If you’re angry or upset, you say you need 30 minutes, and the other person leaves you alone for 30 minutes, and then you hash out the argument. No screaming, no yelling, no name-calling. If we expect that much good behavior from six year olds, then we have a right to expect it from each other.