Poly requires a very strong sense of self, a lot of self-confidence and flexibility, a sense of humor, and a willingness to be honest, forgiving, and understanding. If you don't really like yourself, first and foremost, then your poly relationships are likely to be fraught with drama.
It's one of those remarks that seem obvious to me, but seemed to be a serious revelation to a lot of people. And that surprises me, until I stop to think about it. Because there are many people out there who make the mistake of thinking that relationships will define them, instead of being just a part - though an important part - of their lives.
Relationships are wonderful things, but in order to be successful at them, it's important to have a good sense of self. Without that, it's easy to either lose oneself in the relationship or to become so insecure and demanding that one drives a wedge into the relationship.
Disappearing into the relationship is how people find themselves wondering why their partner treats them like a rug. Even if the partner is by nature a loving person, it's human nature that we get busy and preoccupied and to fall into a pattern of taking unnoticed advantage of the conveniences around us. If a partner lets us treat them in that way, even the best of us will be guilty of ignoring unspoken needs. Turn that partner into someone who is self-centered or strong-willed, and becoming a rug is easy.
Now multiply that by several partners. You can see where that gets ugly.
Likewise, being insecure and demanding is just asking for continual drama. In a monogamous relationship you are not going to always get your own way, or get all the positive reinforcement and attention you want. In a poly relationship? The number of needs and demands - relationship, life, family, personal - on the relationship grows exponentially. If you can't stand not to be the center of attention, or if you are intolerant of schedule changes, or if you always have to be first in everyone's attentions, then you are asking for poly drama and failure.
Poly is not the place to prove that you are the sexiest, most fascinating, most amazing person in the world. Poly is not the place to salve one's pride. Because there are going to be times when partners are busy with the other aspects of their lives and their loves will not be number one. Even for primary partners, there are times when work or children or parents or other demands are taking the other partner's energy and attention. Someone who can't accept that they are still loved, and can't distinguish the difference between a temporary distraction and a serious relationship problem - and can't discuss potential problems in a calm and adult way, because there are times when you have to check in - will find their poly relationships are going to be fraught with crisis and anger, and are likely to find relationships ending without really understanding why.
If you are looking for poly to justify your existence or make you feel good about yourself, then there are times when it's going to let you down and let you down HARD - not because anyone is being mean or intentionally hurtful, but because Life Happens. And if you can't take Life Happening without falling apart, then you will generate poly drama. Which is likely to spiral out of control.