With the many careers out there, it is a matter of choice and motivation for picking one.  Enthusiasm to work for a certain profession comes from many different reasons.  For every person, certain motivations can be meaningful or lack in importance.  The stimuli for a certain job can come from: money, contact with other people, feeling of importance, creative satisfaction, & interesting challenges and intellectual stimulation.
Money surely is one factor.  It is important because money makes the world go round.  You need money for everything in this world, including the basic needs of food, clothes, and shelter.  Money is also needed for gas to go places (like a job) and money to go out to places.  However, people may want a job because it earns more money and they want to be rich.  This permits them to do (as long as in reason and abiding by the law) go where they would like or get what they desire.  Of course, some people have really high ambitions and money is what they need to get where they want.  Money is also needed if you want to run for a political office like Senator or President.
Possibly, money is not such important to a person and what drives them to go for a job is contact with other people.  Since no two people are the same, there are individuals who are more social then others.  People might like to get up in the morning or nighttime, for a job to meet and talk with the other employees.  These shifts (like the night shift of a day shift) can contain people who you really enjoy being around or talking to them.  Or sometimes the profession involves meeting new people and talking with them.  An occupation like this could be a telemarketer or being a flight attendant.  Even as a Medical Assistant you can meet new people.
Another factor of why someone would want to go to work every day is the feeling of importance.  Someone who has a political office probably feels vital for they have a position of authority and they get to make decisions that can affect the people around them.  For some though, they are content to not work in a certain career because they don’t want to make decisions that citizens would lecture them for.  It happens all the time, just look at the anger of the people at the U.S. Government currently dealing with the Hurricane Katrina.  A feeling of importance could also from getting a job well done and being appreciated for it.
Still, for some people, creative satisfaction is a characteristic that appeals to them in a job.  For me, creative satisfaction is significant, since I love to work with the Fine Arts.  I am truly pleased when I get to draw, color, or even do artwork on the computer.  Some of this satisfaction can come from when people are awed with the work you have done and go “o” and “ah.”  It also comes from a job well done when a client thanks me and is awed as well.  But some jobs do not need this, like a Medical Assistant.  As a Medical Assistant, you are working more with people and medicine than the Fine Arts.
Some people prefer jobs that challenge them intellectually or that are interesting.  They, perhaps really enjoy challenges or doing things differently.  When a job is tricky, it can be exhilarating to accomplish a task set out to you. An example can be like this is: you have been given an art commission that is unlike what you have done before.  It might entail you to look up and learn new material to do what you have to do.  Then it requires applying what you know to it and then perfecting it.
Everybody has different interests in life.  Each person can feel a calling to a certain career because of there reasons, which motivate them. So why one gets up to go to a job, whether it is day or night, all depends on the motivations stated previously.