The question is in what country, cause different countries have different mentalities regarding the defense service. In the US many people see it as a career opportunity and a way to get a new start to their life. Others join, lead by their ideals of fighting and defending one's country.
In some countries defense service is mandatory and therefore people join the military without much of a choice. In these countries you can also find many people joining the military, driven by the consensus. Knowing that everyone else does it helps you continue in their path and sometimes even makes you want to be no different than everyone else.
