"What, aren't I enough for you? Aren't I good enough?"
When this question is asked from within the "paradigm" of monogamy / monogamism, it is taken for granted that one person should be able to supply all of one's most intimate companionship -- to complete satisfaction --, and that dissatisfaction with this purely dyadic "picture" of love must indicate one's partner in love is failing and inadequate.
The poly view is that this need not be the case at all. From this perspective, it is unreasonable to expect one person (any person) to be "everything" to any of us. We generally acknowledge this in the arena of friendship. Most mono people would not prohibit close and intimate, albeit nonsexual, friendships. But why limit loving relationships in this way? Why the expectation of sensual/physical/sexual exclusivity?
Poly folk will generally interpret this expectation as a barrier to intimacy with others rather than as some sort of sanctification of a relationship. Mono folk will generally consider physical exclusivity a sort of sacred oath of "true love".
Saying so doesn't mean voluntary monogamy is "less than" poly. Poly folk generally have a live and let live attitude, wheras most mono folk tend to think their approach is the one correct one.