Unless the monsters are
extremely dangerous, we fight. (Wolves aren't that dangerous to even low level PCs, especially if they're picturing, in their minds, what a wolf can't do to someone wearing armor.) Fighting gives XP. Fighting clears opposition from your back, like when you're leaving the castle. Suppose the DM said that you get more XP for avoiding certain encounters than for fighting. Okay then, avoiding the wolves is so easy you don't expect XP for it, so we fight.
If the DM had described them with red eyes (or some other hint that they might be werewolves), the PCs might run. Or maybe scale a tower and loose some arrows and spells! (More dangerous? More XP!)
I've found low level PCs are, of course, more likely to run. They'll run from a pack of 1st-level characters they could easily defeat if outnumbered 2:1 - there's no way for them to know how dangerous humanoids are.
ThirdWizard said:
This means that the DM has to encourage it by 1) using encounters that they can't beat giving them a reason to run and 2) not always chasing after the PCs to kill them.
Unfortunately, #1 often ends in TPKs.
It's not easy to make to make it obvious you should run, unless you use something scary like animated bones (wolves aren't scary to armored or spellcasting PCs). Furthermore,
when should you use such encounters? Scattered randonmly 5% of the time? Only for the boss' lair, until it's time for the PCs to attack? The ancient evil's lair, which isn't part of the current adventure(s)?
There are few cues for DMs and even fewer cues for PCs.
A lot of adventures should have you run and not even attempt to complete them. I'll give two examples:
Grasp of the Emerald Claw. Taking on the temple at the end required several days of campaigning (both in and out of game). It was obvious the DM was being nice to us - we were never attacked in our sleep (we set watches, etc, though), despite the presence of intelligent opponents
who knew we were in the temple. Really, we should never have gone in there once it was obvious we couldn't get what we wanted in only one day, especially for the reward (gold). It's a 6th-level adventure, so no teleporting, and my character, the mage, refuses to use broken spells like
rope trick - not that he had the slots to devote to it anyway. (I could say almost exactly the same thing about SotW or whatever it's called - the one where you go into the Mournland. There's no way we should have faced only one random encounter, which we
did run away from, seeing how any monster in the Mournland is mysterious and you can't heal, either.)
Another example involved an adventure in Modern (a published one) - the PCs were storming an enemy camp. The number of opponents in the camp was very high, although most were of low quality. There's no good reason, other than player enjoyment, for the GM not to take half the NPCs and have them attack the PCs simultaneously. The PCs would simply die - have a fun adventure. Upon realizing this problem (compounded by a fairly poor map), I had to hand-hold them by making the NPCs deaf and dumb.
In most Modern adventures, I found it just a bit easier to get the PCs to avoid fights. You're not powerful enough to take on four encounters per day, which I'm thankful for. (Coming up with four plot-relevant encounters in a single game day is very difficult.)
I think players get used to hand-holding - if they weren't, they'd never complete a lot of adventures.