I think there should be a basic desire to help people, and I think you need to have a true interest in science. Without those two things you will hate your life and you won't succeed at anything. However, you can't completely forget about money. Although most people in the pre-allo forum don't know it yet (you lose most of your idealism during your first year and it keeps diminishing during every test cycle), money is a very, very important factor in this equation. That's one of the reason doctors make very good money. It takes a lot of reimbursement to get people to work as hard as med students/residents/physicians are required to work. Just look at how many people are willing to become FPs. To many students the pay for an FP, while still well over $100,000, might not be worth the hastle and hours you'll work in practice.
These comments about there being easy ways to make money are bogus. If there were an easy way then everyone would be doing it. There are very, very, very few I-bankers. Most lawyers won't make what physicians make, and the ones that do are either the scum of the earth (mass tort lawyers like John Edwards...of which there ar every few) or >10 years into their career. There are A LOT of lawyers out there making resident salaries due to the overabundance of law schools, so unless you graduate from a top law school you're going to have a tough go at it. MBAs are the same way. There are now so many MBA programs out there you have to graduate from a top 20 program just to have a chance at starting out in the six figure range.
If you can make it through 4 years of med school and 3-7 years of residency (which is very tough, but most do it) then you're set. As far as average salaries, give me a surgeon over a lawyer or MBA any day. The job stability and options of a physician are unparalleled, and they will earn a salary that easily ranks them in the top 2% or 3% of all workers.