"Bonding" is, in most human societies, as a really good thing. We celebrate marriages and friendships all the time. Being dependent on someone else, though, is a bit more troubling, because it implies that you cannot manage on your own, which is bad because it means that you can't end the relationship if it becomes more harmful than beneficial.

But "co-dependency" is a different thing than dependency. The word looks like it should mean "two people who depend on each other," but that's not what it is. A codependent relationship is a relationship that is severely uneven: one party primarily benefits, and one party primarily suffers. Imagine a relationship with a husband who sits at home, drinks, does not clean, does no chores, does not work, does not help with the kid, just sits and watches TV, and a wife who has a job, does all the chores, raises the kids, feeds everyone, etc. That's a codependent relationship, and it's a bad thing (except, of course, for the beneficiary). Some people have what's basically a disorder, in which they will actively look to get themselves into these situations as the caregiver, and they can be described as "codependent" or having a "relationship addiction."