You could argue that you're not required to, but then again, no one is required to read and answer your question.
A lot of users, myself included, tend to ignore questions which are purely a problem statement. In the past I have given answers to such questions, only to find that the user was still confused about some much more basic concepts, making my answer pretty much useless to them.
It is incredibly frustrating spending a lot of time on an answer, making sure it is well thought out and presented, only to spend even longer discussing the solution with the OP who could have saved us both some time by following the advice given and including their thoughts on the problem.
Adding such details usually helps the OP clarify and understand the problem. I myself spent fifteen minutes tonight writing a question for this site, only to figure out the answer, and therefore did not ask the question; this occured because I made a special effort to clarify what I was confused about.
I imagine you would downvote a response which contains just the answer to a problem you've posted as opposed to a detailed solution, the reason being that you want to understand the solution. At least, I hope this is the case. If you want to understand, the first step is to make sure you understand exactly what it is you are trying to understand.
The take-away message is this: including your thoughts and questions about a problem helps you to better understand the question, and it helps everyone else to answer it appropriately.