I'm currently running into an issue with my plot, specifically that I have an idea for how I want the game to play and my plot has taken a direction of its own and is veering away from fighting and running into a quagmire of very little physical fighting and lots of political drama. So, both in the context of my particular problem, and in RPGs in general, what are some valid reasons to fight? Monsters randomly roaming outside town just doesn't cut it for me.

Your character is getting into politics? Sounds like someone's going to try to assassinate him/her to get rid of the trouble.

And of course, that would warrant some badass retaliation...

You are a monster and have a instinct to kill people
You want to defend something or someone from something or someone that might cause them harm
You are a mercenary
You are in the army
Someone said C was better than C#
You have a bomb in your head and if you don't fight it will explode
You are a professional fighter
For fun

You have a lot of testosterone
You are drunk
A drunk person attacks you
You want to take out revenge on someone for something
To escape

I'm not sure what you mean by "valid reasons to fight." There are many reasons to fight, and not all of them are valid. Whether they are "valid" or not depends on laws and morality.

The question is a little vague.

Do you mean excuses to make the player engage in combat?

Excuses to fight, but with more substance than "every creature outside town hates you so go kill them all, oh, and there is a dragons lair right over there"

You need to be a little more specific and give us at least a logline worth of plot. But, we digress. Of course, money is always a good reason to do something. Perhaps your character as no real way of sustaining themselves and thus has to rely on money from "bounty hunting" to survive in his world. Maybe he's seeking fame for "slaying the dragon" or something similar. And of course, unresolved rage is always a good one. Perhaps something happened in your character's past which leads them to live a life filled with anger, searching for resolution (Mad Max). Another possibility is that your character is just bored with their everyday life and is seeking adventure (Life Aquatic).

Probably the more important question you should be asking yourself is what kind of character your protagoinist is. As you can see, nearly every reason for fighting is derived from motivations that deal with what kind of traits your character exhibits. Whatever kind of character you have, they should have a flaw (or encounter a physical problem in the world) that can only be solved with physical intervention (fighting/violence).

For example, in the Mad Max films, the main character is filled with rage which can only be resolved by causing physical harm to those who cause harm to others. Thus fulfilling his need for redemption (saving others when he couldn't save his family) and his desire for order (delivering justice when there's no laws).

So, once you figure out just exactly "who" you want your protagonist to be, you should be able to figure out what motivates them, and thus determine their reasons for fighting.

so if I understand this correctly, you want to include more combat, to avoid your game being bogged down by the complexity of the political system? if so the below might help a bit, and if not, I would definitely suggest clarifying what you want to find out, as it is at the moment a bit vague.

well, I'd say that the most important choice in the type of conflict is whether you make it personal or global.

Personal would be that either your or bob the bow salesman has a grudge against those pesky orcs outside town, because they are putting him out of business, since he can't get wood for bows and arrows, and wants you to kill 10 of them, or retrieve 10 orc topknots, or axes, or whatever as proof of this. Or the orcs stole some of bob's stuff and you go kill them until the item 'bob's stuff' drops.

or for more serious examples, you as a member of a certain political party are entrusted with the task of incapacitating a member from a rival party, through deceit, murder, or any other means you can devise, and this task was given to you by your mentor within the guild/political party, and is largely a personal reason to fight, to improve your standing, or to prove your worth to your mentor.

Or you have global conflict, ie. the human nation is under serious assault from the neighbouring troll tribes, and as a drafted member of their army, you have a quota of trolls to kill, and you won't be given your daily ration of ale and cheese if you don't.

and again a more serious example would be that you nation you belong to is fighting a different nation for control of a colony, or other expansionist conflict, or your nation is being assaulted by another attempting to annex you, and you are part of a resistance force, giving you reason to go kill some of those pesky invaders, which are probably at the moment only a scouting force, justifying solo/party play.

There are of course advantages to both of the above, but I think on some level it can be quite important to include both. While your main story arc may be based upon global conflict, I think it can be quite important to include valid personal reasons for conflict, to avoid existential crises in the middle of the game. And I agree with you when you say that the presence of orcs doesn't justify their relentless slaughter, however it is hard to create an umbrella reason for combat, and if you want to avoid generic reasons like 'you have an inhumane hatred of the inherently "evil" humanoids' or 'your king needs you to save his kingdom!', you have to create them to fit your characters, and your story, which Sulphix has also mentioned.

I'm sure people here would gladly continue to help you, it is just very difficult to avoid cliches in this case without knowing a lot more specific details.

The hero's religion says that you need to kill monsters to go to heaven (or something like that). [grin]

(This might look like a dumb idea, but if you develop it, it could become interesting)

I'm pretty sure I'm going to just change my plot completely because it is drifting away from the gameplay I wanted and to fit my vision into the what my plot has grown into would feel forced. I dunno what I will do with the plot, but stepping back from it, the conflict in it feels forced, which is why elevating it to physical combat is difficult.

ok, well it's better to discover these things while still in the design process, so best of luck with that. And post your modified plot idea some time if you want some critique, as people around here tend to be more than helpful when it comes to things like that, and personally I'd be quite interested to see what you come up with.