The example you give is not about the active voice versus the passive voice. "To be committed" and "to commit to something" don't quite mean the same thing. As the President, if I am committed to reforming heath care, it means that I am dedicating myself to the cause and will do my best to make it happen. If I commit to reforming health care, it means that I promise to reform it. Active and passive voice work like this: - The cat eats the mouse. The mouse is eaten by the cat. - John Smith wrote this book. This book was written by John Smith. - I will drive you home. You will be driven home by me. Why is the passive voice often preferred in English? Good question. In the examples given above, it's to emphasize the person or thing acted on. We'll say "This book was written by John Smith" if we want to talk about the book, and say "John Smith wrote this book" if we want to talk about the author. The idea is to focus on the action rather than the person doing the action. In my examples, we can completely ignore who is doing it and just say "This book was written in 2010" or "You will be driven home later on". This can't work if you use the active voice.

"The president is committed to reforming health care" is not passive voice. The word "committed" is an adjective here. In actual passive voice, the doer of the action, stated or implied, follows the preposition "by". Passive voice is used when it is not important or we do not care who did the action, or when a writer wants to emphasize something.

"The President is committed to reforming health care" could also be said, "Health care is what the President is committed to reforming." In the first phrase, the emphasis is on the president and his commitment; and the emphasis in the second phrase is on health care being reformed.