I think a lot of people have been "overly invested" during various points in their lives -- hell, I was attached to Ender's Game while being violently bullied on the grounds that, even though we were all too young to really be dating, I was a lesbian and that's contagious -- but... even when that book seemed to tap into something I was going through, I still knew it wasn't great literature; it was just a book that hit me at a certain point, but that didn't make its flaws invisible.

There's nothing wrong with seeing yourself in a work or being invested in fandom, but it's the soap opera hysterics and inability to admit that, at the end of the day, SPN is not a cultural milestone or anything but a surprising cashcow and that parties involved are freaking out over ... a plot that was spoiled months ago but fandom was in denial about? -- that's what takes this to the absurd.

I was devastated after finishing Persona 3 FES (even though I think that game is perfect) but... there are people who know why that game would hit me like that at this point in my life, plus my life is limited because of a disabling illness and that game kept me going. Over investment? I don't know. Possibly? It's when the "legitimate, if personal, reaction to a work" gets buried under "fandom hysteria at stupid shit" that things cross the line for me and "over investment" is as good a term as any.

The problem, I think, is that while you named an example of being (maybe) "over-invested" in a manga, SPN fandom is over-invested in their fake canon that will never come true and is full of problems, but OMG IT WOULD BE REVOLUTIONARY SJ TV... which is where the cognitive dissonance and "HOW ABOUT NO" sets in, wank-wise.