There is a pretty common pattern I've seen within poly relationships that is also seen in the mono world. The shopping up concept. This is where one shifts from one relationship to the next, based on what feels best at the moment. Serial monogamy is an example of this, but I've seen it in poly as well.
There can be cracks below the surface of a connection that are glossed over, right up until the point where someone "better" comes along, at which point in time, it's all up for grabs. After all, who wouldn't want to be in a better relationship? To experience a more amazing connection, conversation, sex, and love? It seems that this tactic, where one is looking for the next "best thing ever", is counter to some significant common relationship skill goals. Chief among them is exploring what a sustainable relationship looks like 10,20,30,40 years in.
Many of us still visualize our golden years as a space that we share with partners we love and care for, nurture each others growth, that we have comfort with. By consistently moving from relationship to relationship, no matter the number of partners involved, we cut ourselves off from the opportunity to learn the skills that can get us there. How do you stay involved and vested in a connection that doesn't have that same "spark" it had 20 years ago? How do you create a sense of excitement discussing things with someone you think you already know like the back of your hand?
Some of this is about choices on each person's part. Do you seek out shared experiences? Do you attend to your personal growth in addition to your presence in your relationships? Are you as healthy as you can be physically, mentally, emotionally, sexually, professionally, and spiritually? Do you look to create a sense of kinship, community, or family in addition to the flashy parts of your relationship? When is the last time you spent as much time on foreplay with your long term partner as you do with your newest one? Looked to create a date that would be meaningful to them? Asked them about what they feel would improve your connection? Do you nurture yourself regularly with activities that feed your sense of self?
When the lust for change is coursing through your blood, find a way to manifest that change in your current relationships and within yourself. Don't close yourself off from new connections, of course, or stay in a bad situation, but just moving onto the next best thing is the path of least resistance. Even if the relationship ends up looking pretty fundamentally different than it started out as, that is growth. The nice thing about poly is that we don't need to have every relationship look the same, or satisfy all the same needs. We can make it be whatever serves the needs of the people involved, and that is what is important in the long run, however long the run may be.