Maybe you misunderstand the purpose of the Subscribe function.
If you ask a question, you will automatically receive an e-mail notifying you of any answers you get.
If you simply answer someone else's question you get no further feedback.
If a particular question asked by someone else is of interest to you, you can "bookmark" it by ticking the "I want to susbcribe to this thread" box. You can then easily go to your subscribed threads later to see what replies have been given. However, if you give an answer yourself, you do not need to subscribe to the question because the thread will be shown on the "Threads I'm involved in" and the "Q's I gave answers to" sections in your profile.