In an organization as diverse, multifaceted and accepting as Starfleet, there’s bound to be a variety of different reasons to make the commitment to the fleet. For some, it’s the mission itself. More noble goals than Starfleet’s are difficult to come by, and their pursuit contributes to the expansion, continuation and improvement of an astonishing interstellar civilization. Other characters might have joined out of a sense of familial obligation. The fleet has been around in some form or fashion for centuries, and a tradition of service exists in many families, sometimes reaching back three, four or even five generations. Some might feel a desire or a need to protect the Federation, especially considering the many conflicts and dangers that exist in the not so empty depths of space.
Other, less personal reasons might have been an impetus as well. Serving aboard a starship is an excellent way to get the feel for a society, and appreciate its positive and negative aspects. This is especially true with newly discovered species, or those that wish to be the first of their kind in a Starfleet uniform.
The reasons are, naturally, varied, and can say a great deal about a character before we even get to know them. So what was your characters’ impetus to join Starfleet? Was it a desire to contribute to the Federation and be a part of something bigger? Was it to protect and defend? Explore? Something not mentioned here?