There are, from my limited understanding, a vast number of reasons (The degree to which each applies varies depending on the individual) as to why people feel it is honorable to fight in a war.
The first is the mentality that sacrifice=good, without any further elaboration on sacrifice to what. The desire to suffer and sacrifice, the willingness to undergo such circumstances, is a perminant aspect of human nature, and will never go away. If one simply remembers that the state perverts everything that it comes in contact with, one can understand how this natural “inborn” part of human nature is used to make war honorable. The willingness to undergo suffering for a greater cause is certainly noble, but the nobility is dependant on the greatness of the cause, not the willingness to suffer. This isn’t to say that individuals currently fighting for not-so-great causes don’t have the potential, merely that such a potential is currently misdirected.
Another common underlying cause of the aformetioned belief is the individuals need (yes, need) to identify with a group, and the desire to protect that identity (personified in this example as the “Nation State” while in a free society individual idenity is likely to be much more complex and less violently expressed) When the subject of war is considered, one imagines that one is fighting for one’s country (An entity that cannot be described very well by much anyone, hence the problem of it’s use) it is merely an extension of the “self defense” instinct, now applied to a larger “self”. This instinct will not disappear, nor ought it. Humans form groups and protect them for a reason, beacause we need to. However, this part of human nature can be perverted, as is easily demonstrated.
Besides this, there is the fear of feeling otherwise. People, who identify themselves with a “country” now are partially responsible for “it’s” conduct. When something bad happens to an American, it happens to “America” and "America "responds. This collective individuality (a contradiction if there ever was one) creates feelings of pride and guilt. If someone has a choice, will they feel pride or guilt? They will choose the former, as opposed to the latter.
Of course, there are the most base of motivations, the desire to fight and kill without consequence. However, it is my belief (being that I have no reliable statistics on the subject) that this is a minority amoung those who currently serve the state (Although it could be dispropotionatly large in comparision to the general population, as I said, I woudln’t know). From some degree of experience (an experience colored by various factors including distance, emotional investment in a postive conclusion, etc.) I can say that most people don’t hold this as a top priority, if at all.