Here's something I've noticed in several instances, both personally and by observation, and was wondering if others have observed something similar: When a couple is very well pair-bonded, it can feel intimidating to another party entering their sphere. This may be one of the reasons that it's tough for many that are "looking for a third" to find what they are seeking. It's just exceptionally tough for someone that is becoming part of the space of an established couple to feel that they are ever going to catch up.
On the other hand, there are people out there that crave and delight in sharing the energy and space with a well-bonded couple. It feels safe and welcoming, and like they can piggy-back on what has gone before to advance farther and deeper more rapidly.
Speaking personally, I've been on both sides of each end of the equation. There have been times where I've been the third party in the room, and I've felt either isolated, or cherished. On other occasions, I've seen someone either feel the rush of sharing in a bond that they can feel from the outside in, or seem pushed away by the strength of it.
What is it about strong pair bonding that can bring out deeply held fears of adequacy, or enhance the joy of togetherness? Are we hard wired for pair bonds, or is that strictly a societal construct? For me, I've been in strongly felt pair bonded relationships since I was 18, and that seems to be a deeply held desire for me, to the point of almost being a need. I don't think I'd be happy occupying a more peripheral sphere in all of my relationships, but enjoy deep emotional and physical intimacy with others in addition to my main pair bond(s).
Many of the most independent personality types that I've run across seem to be content to observe the pair bond in others, or participate from a support position. Often, these people are happiest to have connections with others that aren't as entwined as live-in partnerships, but still enjoy a deep sense of intimacy with others. A pair bonded partner can be an excellent fit for them, since the PBP isn't typically interested in having an additional relationship that involves the same level of interconnected relationship with another partner.
The sticky point can come when a pair bonded relationship ends, and the partners desire their other connections to further develop to pick up the slack. Sometimes the space is there, and other times it is not. If it isn't, that can be rather disappointing to all concerned. In my ideal world, the existing connections would be kept viable while the partner in need of a pair bond is looking, and preferably while they are growing their new relationship. I'm a fan of "grandfathered in" relationships that are still serving a purpose for all concerned.
I consider myself very fortunate to have enjoyed contiguous pair bonded relationships throughout my entire adult life. In the past, I feared being alone. At this point, I am very content with my own company. Poly, by the very abundance that I've explored, has helped to diffuse my own insecurities about loneliness. A person can be utterly lonely in the midst of a crowd, or filled with a sense of companionship with nothing but a good book for company. Pair bond or no, poly in thought or in practice, I'm satisfied.