Every day, civilians across the nation thank military service members and Veterans for their service. The question often arises as to whether someone should offer thanks.
I would begin by highlighting the obligatory nature of asking whether or not someone should say something. Obligations of this sort are contradictory to the very purpose of the service they are being thanked for. The freedom to speak as you wish necessarily also implies the freedom from forced speech. Of course, that's not what people mean when the ask this question...
As to the question of whether it is appropriate to thank a Veteran for their service, I would answer in the affirmative. I recently wrote about the transfer of burdens that takes place between the military and civilian community. The burdens of service "include the loss of personal freedom, physical harm, psychological trauma, emotional suffering, and relationship hardships."
Veterans voluntarily assume these burdens so the civilian population is not required to do so. When a Veteran is thanked for their service, it is much deeper than the typical understanding of the phrase. The core of that gratitude isn't simply "for the freedom." It is an appreciation shown to the warrior class who have assumed life-altering circumstances in place of others.
More appropriately, the gratitude is the simplistic form of "I appreciate that you gave up your personal freedom so that I may have personal freedom; that you subjected yourself to physical harm so my body would remain in tact; that you took all of the negative psychological impacts for me so that I may sleep soundly at night; that you are suffering emotionally today for my emotional well-being; and that you gave up time with your family so that I may have time with mine."
For these reasons, it is appropriate to thank a Veteran for their service, but it is not required and never expected.
[1] I understand that law enforcement officers, firefighters, etc. are also thanked for their service to the community. You may include them in your reading of this essay despite only Veterans being expressly stated.
[2] This is based on the idealized form of military service and does not take into account personal opinions of individual operations overseas.