Generally, for the USAID foreign service, common motivations I hear are some combination of the following:
· Monetary benefits
· Living overseas
· Government job
· Family benefits
Contribution to the big picture
· Advancing US policy and interests
· Improve the lives of others throughout the world
· Representational responsibilities as Foreign Service Office, Consular Officer, and Secretary in the Diplomatic Service
· The specific type of work and assignments
· The camaraderie and interaction with colleagues or counterparts
· The change, the challenges, and the diversity of problems
For most companies, the story is often very typical being it's a well-known company, benefits are good, or they applied to jobs and this fit their field of work. While the foreign service has those same motivations, it's unique in the mission of the organization in forwarding US foreign policy and representing the US government overseas.
Like most, my path to this career is a mix of all the categories. The compensation is competitive, working overseas has been an interest since college, and it's nice having the security and backing of the US government as an employer. The change in assignment typically every 4 years (exception for initial 2 year assignment and 1 year Critical Priority Country assignments) is nice and will keep work consistently interesting. International affairs has long been an interest and the idea of serving and representing the country seems prestigious and worth responsibility. And the USAID focus of making efforts to make a real difference through our work in the countries in which we are stationed is very fulfilling.
I would guess about 50% or more of USAID FSOs served in the Peace Corps. I didn't take the opportunity to join (though I did apply and considered it) but instead thought I would work in business overseas. But after studying a Masters and having a spouse working internationally, I was pretty much set for the past few years to eventually lead to this work.
In human resources, compensation/benefits is key to attract and retain talent. Intrinsic interest in the work and assignment is also important. Additionally, most HR studies will indicate the shared commitment to a common goal or mission is essential to get the highest productivity and involvement. I think the FS really has the possibility to easily have all these traits.
*Note, the job is not all roses and there are definitely downsides in a FS career to consider, top ones that come to my mind are: distance from friends/family, spousal employment/life for spouses, less control over one's life, and separations from family/spouse/children.