In order to better understand the reason that we should not express our negative emotions—that is, desires—outwardly, we need to understand that outward action has consequences. The Buddhist concept of karma is deeply tied to outer action, and says that outer action has long-term consequences that extend throughout many lifetimes. Swedenborg had a similar idea of outer consequences in the sense that he said that the future of one's spiritualized (or, in most cases, non-spiritualized) Being was entirely determined by the nature and quality of its outer action: that is, seen in more Gurdjieffian terms, every human being has a responsibility to act outwardly in order to actualize the results of inner Being.
One cannot, in other words, be passive and just do nothing in the spiritualized state. One is required to do more than just think about the good; one must embody it. This idea of the necessity of outward action is ubiquitous in religious understanding, in the sense that the good must be expressed, not just believed in. And it dovetails quite neatly into Ibn Arabi's belief that we are all vicegerents of God, that is, agents appointed to act on His behalf.
The idea of retaining and consuming our negative emotion inwardly is deeply related to Gurdjieff's concept of intentional suffering. We must be how we are and what we are; but we have a deeply spiritual, not just outwardly moral, responsibility to keep that egoistic and selfish set of desires, impulses, and emotions to ourselves, and suffer what we are. We do it intentionally because we are aware of how we are, we see it, we understand that it is egoistic and destructive, and we actively say no to it. This requires a kind of presence and intelligence that is usually not active in us, even if we are spiritualized. That's because being spiritualized — having a modicum of inner awareness — is not enough. Although many threads and roots can grow in Being to connect us to the idea of the good, they must become strong, and they only become strong through exercise, that is, practice. We must make a real inner effort to go against what we are, and that always involves Being what we are— not being some other, better creature.
I have, in other words, a responsibility to see my destructive behaviors and, in so far as possible, continually own them myself, rather than putting them on other people. This responsibility has been described in detail by many spiritual masters, but I think that the conjunction of Swedenborg and Gurdjieff, along with the important adjunct material Jeanne de Salzmann provides us with in The Reality of Being, provides one of the best comprehensive understandings of exactly why this is such an important practice. And, while Swedenborg provided the essential data illustrating why such a struggle is needed, Gurdjieff's concept of intentional suffering captures the nature of the action better perhaps than any other description.
Each action that I put outward into the world that is selfish and destructive is an action I become responsible for; but each one that I keep inward within myself and acknowledge as part of what I am becomes a pearl of value. In refraining from destructive outward action, I serve the good, because serving the good must always begin with not serving the bad. This is a subtlety that often escapes me, because I believe that I can serve both — and that delusion arises, unsurprisingly, from my desire, that is, my egoistic belief that I can have things the way I want them, and thus serve two masters. This is the way in which selfishness perpetually seduces me; and each act of intentional suffering signals a beginning to the end of that romance.