Loneliness is, indeed, a by-product of isolation. Loneliness should not be confused with being alone, however. It's very possible to be alone physically, and not feel either isolated or lonely. Just as it's possible to be surrounded by others and feel utterly isolated and lonely.
Humans are social creatures. We innately crave connection. From the day we come into the world we pursue connection. We're hard-wired to do so.
Good things happen when we are connected. Bad things happen when we are not. Pretty simple.
Loneliness is, as far as I'm concerned, one of the most difficult to bear experiences we can encounter.
None of us choose to be lonely. None of us choose to be isolated. If you ended up isolated, Laura, it was because you must have felt it was not possible to connect with the person/s you wanted/needed to be connected to. This can happen for myriad reasons, none of which would be your fault. It's possible they may not be anyone's fault. Communication between human beings can be a real challenge. It's also possible that someone does bear responsibility for the disconnection.
Although we deeply desire connection and contact, we must be thoughtful about where we look for it. Not everyone is capable of connecting at a level we may want or need to. Part of developing relationships is going step by step (do any of you remember that "flow chart" image? It's very useful for this) and assess (yes, scrupulously honestly) if the other person can connect in the ways we want to.
If we could knock out isolation in all its different forms, we would at the same time knock out loneliness. Since we're on the topic, it might be interesting and useful to continue to identify ways isolation manifests- the more aware we are of them the better we can avoid isolation, right? I think right.