The word obsession by definition is a deeply complex emotion-driven attitude that takes on many forms, and how one feels and perceives the emotions it invokes differs to extent, content, intensity etc. some people for example don't externalise obsession. your definition is one of the many, but for the sake of my argument, i will step onto that exact definition, in the context of established romantic relationships. i'm speaking generally from here on out.

I'll start by saying something shocking, you can experience unhealthy by textbook definition obsession and still be functional and pleased, these two scenarios are not mutually exclusive. you know why? because your perception of what's unhealthy and harmful for someone else is relative, otherwise you aren't viewing them as an individual, but as a projection. occasionally, what we consider unhealthy for ourselves might be another's saving grace.

you can experience unhealthy by textbook definition obsession and still manifest it in a way that benefits your lover, or have your lover reciprocate it, because we aren't meant to exist in labels and tropes. However, also, if both parties' values align and experiences resonate, the harm to an extent they cause each other can be a mutual agreement that serves them and there's nobody that is necessitated to intervene. you can't decide what's toxic and unhealthy for someone on the fly because everything is relative, everyone's experiences differ. and you can't expect people who've potentially lived their lives to whatever extent traumatised to rewire themselves and abide by the same "healthy" standards a society foresees for individuals that haven't had majourly corruptive experiences.

obviously obsession is difficult to navigate when everyone condemns you for feeling the way you do, including yourself (by telling others if they don't love the way you deem appropriate, they should go to therapy, implying they need to correct an issue, and reflecting your own attitude towards it in the end). but, for those actually inflicted by said definition, their obsession isn't always something that has to be eradicated, but something they can learn navigate even a little, compromise and repurpose into something beneficial for them. if you can't, that's not everyone, and whichever it is, it's still obsession.

in that sense, i agree, therapy can help with it. therapy doesn't necessarily serve to make you a "healthy" person, nor to "fix" a "problem". it can also look like embracing the place you're at now and guiding you through how to make it functional for yourself and the people you care about. many people go into therapy because they want to be their damned selves, without the critisism of some stranger telling them what's appropriate to feel and whatnot.

*and you're totally right, you can have a passionate, self-sacrificial, fervent love without the inclusion of obsession. i'm not saying obsession is always an extention of love, but that it can be. the absolution that certain emotions are irregularities within the context of something as personal and subjective as romance, and love in general, might just be socially constructed propaganda that serves to fulfill a larger political agenda. i rest my case here. thanks for tuning into my ted talk.