I chose "multiple reasons".
First of all, I love biomedical science (duh! - I'm a biomedical scientist
) and I want to learn as much of it as possible, which medical school will force me to do. As a scientist, I'm confined to a small area of knowledge, but I want to know it all! Plus, I like the whole life long learning aspect of medicine; hopefully it will keep me sharp as long as possible.
Second, I'm looking forward to the challenges of medicine - intellectual (I thrive on it!), emotional, physical, any other kind. I simply can't do easy - I become bored and restless, and nothing good can come out of that. On the other hand, I'm most "on", most enthusiastic and efficient, when I have a full load of work.
Third, not as much financial security as job security, which are not exactly the same thing. Making a lot of money is not my priority, but I certainly want to be sure I can make a living, and with the current research funding situation I can't be sure of that. I'm planning to do some research in my future medical career (consistent with loving biomedical science and intellectual challenge), so I will still be competing for grants etc., but at least my livelihood will not depend on them.
Last but not least, just as wholeheartedly mentioned, that warm and fuzzy feeling you get when you help someone is pretty darn pleasant
Not to mention, I hope I'll feel like I'm doing something more useful than satisfying my narrow scientific curiosity at the expense of taxpayers (this last one is kind of tongue in cheek, but there is a kernel of truth in it anyway).