Well it can certainly be different for different people. If we are trying to find commonality amongst people, and our type in particular, it doesn't appear we are in agreement here. Perhaps it's because there is not a common, driving reason. Maybe people get attached for different reasons, depending on the peoples involved. Maybe we each get different things out of being attached (or over attached). INFJ's tend toward OCD I believe, so this is involved too I think. When we go all in, we go ALL IN.
Yes, I do agree though that we can definitely get TOO attached, and it can often be unhealthy. We need to be aware and draw boundaries if needed. I still say it can have a "feel good" allure to it. Obsession often does. Again not claiming this is healthy, just something that is present.