It's on the person explaining the problem to say whether or not they're looking for a solution, or just for someone to vent to.

In a just world, you would be right. Unfortunately, the way that most people communicate this intent is varrying levels of opaque for us, so we have to do what we can with what we have.

You can choose to try and communicate, or you can get mad when you guess wrong because the other person wasn't explicit enough.

I've found that choosing to communicate (massive_bereavement's "what can I do to help?" is a great option.) is usually the best way to handle this situation.