This is an attempt at a description, more intensional than extensional, of a phenomenon that can adversely affect communities. I'm trying to avoid having anyone specific (especially specific-and-local) in mind while I write it; I just mean to define the phenomenon as a reference point and wave tentatively at some possible modes of address. Content warning: exercise caution if prone to psychological spirals about your value as a person.
Some people are needy. Really, really needy. Not so much in a "loan me fifty bucks" sense, although that can be an inconvenient comorbidity. More in the "stay up until 3am reassuring me that I have value... four times a week" way. In the "sudden escalation to romantic advances at tiny to negligible signs of openness" way. In the "every life setback is a catastrophic trigger leading to a monthlong mental health crisis" way. And this neediness directly reduces how rewarding it is to provide reassurance, romance, or crisis support. And if the needy person finds a source anyway, they're unlikely to improve much before the source is burned out and has to abandon them out of self-preservation.
I've taken to calling the class "misery pits", because you can throw enormous amounts of help and companionship and unconditional valuation into them and they look as deep as ever, but in general it's not paradigmatically manipulative. The need is genuine. They're really suffering! They actually need the things they ask for! It's just that they need insane amounts of upfront investment, amounts that individual humans can virtually never muster and would generally be ill-advised to try to scrape together, before they become able to do normal-person self-maintenance tasks so that any support can stick, let alone before they're able to provide normal-person friendship contributions*. They are not, typically, suited to being paired off in mutually beneficial codependence; they just don't have the resources or skills.
Non-misery-pit humans have a wide range of susceptibility to the desperation of nearby misery pits. Some people can't resist them, or certain instances of them, and pour their lives into trying to help a misery pit; they're so sad. (This is most of the adverse effect on communities I mentioned.) Some people find them repellently pathetic and avoid them altogether. (The effects of this on large social webs are some more of the adverse effect.) Some people are good enough at setting boundaries to avoid throwing anything they can't afford to lose into the pit and can maintain healthy contact with misery pits for extended periods of time. Some people just take a long time to determine that someone is needier than they can handle, and make bad investments and cut their new misery pit friend off after they've come to be relied on. (This is the rest of the adverse effect.)
Misery-pit-hood is not necessarily permanent. It often responds to a change in life circumstances (leaving an abusive household; finding a (usually) girlfriend, who has more love than the pit has depth; the straightforward clinical abatement of a depressive episode). I tentatively hypothesize that at least some could respond to coordinated lovebombing from multiple people each individually contributing sustainable amounts of energy, but have never seen it tried. Failing a way to fill in the pit, teaching people not to dump anything they're going to need back into such a pit seems like the next best thing.
*Some otherwise-misery-pit-shaped people manage social reciprocity more or less competently, but I think they tend to do it in a weird way - more service-provision ("I'll do your dishes") than social ("I'll be intrinsically fun to hang out with").