Great question, SamIam --
My take on it is this. True STO (service to other) beings know that to attach oneself to a human (usually by fusing themselves to the spine, especially around the back of the neck) is not STO. It's an act of violation or psychic possession, which goes against Universal Law. So they don't do it. Ever.
STS (service to self) beings, some of whom might be relatively "nice," can become attachments sometimes through little fault of their own. For instance, discarnates who are confused about being out of body can sometimes connect with humans who have a lot of light, instinctively gravitating towards a person they feel has good energy who can provide them with warmth, and if they get too close, they become attachments. These discarnates didn't necessarily have any bad intentions, they were just clueless in the afterlife, as many discarnates are.
The problem is, though, that the discarnate is then "STUCK" until the human they are attached to starts working with self-clearing meditations, prayers, or other forms of self-healing. If a person with discarnate attachments enters traditional therapy, for instance, what will often come up in those talk therapy sessions are weird, displaced emotions that don't even seem to be originating from the "host" -- but the host has confused that discarnate's emotions for their own. Depression is often caused by attachments, many times by discarnates who have lost their way. Gently send them off to the Light or to the Higher Realms of love, and the depression is gone. This is also true with schizophrenia and bipolar disorder. I've witnessed firsthand with people how entity clearing can heal schizophrenia and bipolar disorder. Seen some dramatic cases.
But if the person has become accustomed to those discarnates as being their "Muses" or their 'Friends," they can actually get LONELY after the entities are removed. And they can welcome them back in again if they aren't careful. This happened to one lifelong schizophrenic I worked with. We had cured him of his schizophrenia, but because he had been hearing these voices of these discarnates for his whole life, he was actually freaked out by the silence. And after a year of being well, he specifically reached out to the discarnate world and asked for more "friends" to take away his loneliness. So some other discarnates came in and attached themselves again.
Whenever we look outside of ourselves for truth, advice, or companionship without connecting with our heart and our own primordial connection to truth or love, we can get into trouble. We can start becoming dependent upon forces outside of ourselves that may or may not have our best interests at heart.
I used to work as a professional actress and singer/musician, and during that time, I realized that most of what artists came to think of as their "muses" were actually very creative discarnates inspiring their various works of art. And there was a clear division between "dark art" and "light art." The artists who were obsessed with imagery or themes of death and destruction invariably had not only negatively oriented human discarnated feeding off their energy (they'd get a big rush as the artist went into a creative frenzy creating their dark artwork or dark music) , but these artists who did dark art also had more extremely negative entities like demons attached of them. No surprise - these dark artists were extremely dysfunctional in their personal relationships and work life.
The "light artists" usually had angels working with them (angels are never attached to a person; they can come within a person's sphere of influence or auric field to help, but they never attach to someone.) Or the light artists might have had mild discarnate attachments working with them, the souls of relatively creative and "nice" dead people inspiring them.
An attachment, though, is always a negative thing because it interferes with the spiritual clarity and well-being of the host. Under Universal Law as I've been taught it, there is a "one soul, one ride" rule. You're not allowed to live out a pseudo-life by proxy, by attaching yourself to somebody else and hijacking their vital force.
An angelic helper or a spiritual guide will never become an attachment. There are many wise human discarnates who are of a higher vibration than most human spirits who will work to help humans at various times, and they are NOT attachments, they simply come in to a person's space when asked and leave afterwards. This is what happens when I consult various wise ones for help with different things. They are often indigenous peoples, the souls of American Indians, aborgines, Celts, or African shamen or priestess/healers.
One very sad and extremely common case of discarnates who remain attached to people through no real fault of their own is the case of stillborn, miscarried, or aborted children. Many women, especially those who have an extremely hard time getting rid of that "extra ten pounds" around the tummy post-pregnancy, still have some of the auric shell of the baby they lost attached to them around the abdominal/sex organ area. There really needs to be a whole protocol developed to help women who lose a child, because even if the child died when they were older (not babies anymore,) women need to be decorded/disconnected from the child, and they very rarely are or very rarely know how to do this. And this creates problems for the spirit of the child, who needs to move on, and for the mum, who can suffer very sad energy effects. The entire soul of a child is not there in utero, only part of the shell from its physical energy body, but that piece still needs to be energetically released. And for most women, they can't do this on their own because of their own grief process or their own complex emotions (in the case of an abortion.)
So a dead child can also be an attachment. Very common, really. Not enough energy workers right now on the planet to address this concern. Also, if you were a twin in utero but your twin died and only you survived, some of your twin's energy will be attached to you, and this needs to be cleared, too.
Another area that overwhelms me personally is the issue of soul loss amongst veterans. I heard yesterday that something like a third of the soldiers coming back from Iraq/Afghanistan are already showing signs of mental illness, and I'm sure the real numbers will be much higher. These guys and girls need soul retrieval to reclaim lost fragments of their souls, and they need clearing of attachments because the military creates lots of demonic attachments that really pollute the soul. And they need other energy healing work. With this type of work, they can be well again. But there are far too few practitioners who are experienced enough to work with these issues, and far too many in need of help.
Anyway, I've rambled here. Suffice it to say that in my view there is no such thing as a positive attached entity, in the sense of one that SHOULD be there. Nothing should be fused to your energy body but YOU.
Spiritual helpers and STO types of beings like angels never fuse themselves to anyone. And that's how you can tell the difference between beings - if they seek aggressive entrance into your energy body or physical body, there is an attempted psychic possession going on. And if they lovingly and quietly offer help on an as needed or as requested basis only, then they are true STO helpers you can trust.
My take, anyway!
LipstickMystic aka Jennifer