1. Your God Abandoned You
Wow, that must have been pretty incredible to MEET a god - well, C3PO isn't an actual god (more of an effeminate robot butler), but the Ewoks THOUGHT he was a god. Some kind of benevolent savior that had been delivered to them for reasons unknown. However, after Return of the Jedi, odds are C3PO went with his buddies and left Endor - and the Ewoks.
That's right - their god abandoned them, never to return. That has to sting - the ultimate rejection - a being that you pray to and honor as the highest of being, just picking up and leaving when you need Him the most. After all, C3PO just convinced them to join the Rebel cause and battle the Empire, which led to countless Ewok deaths.
2. A Bunch of Your Friends and Family Died For No Reason
You know what? The Ewoks were doing juuuuuust fine before Leia, Han, and the rest of the rebels showed up. Sure, the Empire had built a shield installation on their forest moon, but they hadn't bumped up against the Ewoks or anything. I mean, why WOULD the Empire bother to fuck with the Ewoks, an adorable tribe of teddy bears who didn't bother anyone? Even they aren't THAT evil.
Enter the rebels. They convince the Ewoks to join them in their fight against the Empire and their attempt to destroy the shield installation. In the process, the Stormtroopers (who normally miss EVERY SHOT EVER) straight up killed a bunch of the Ewoks, while the rebels got by mostly unscathed. How did this help the Ewoks? It really didn't at all. Their home wasn't in danger, none of them were being killed by guys in white-plastic armor, and they DEFINITELY didn't have to deal with being tricked into anything by a farmboy who could make stuff float.
The only real difference is that a bunch of them are dead now.
3. You Were Almost a Wookiee
Being an Ewok is mostly fine - you're cute, you don't have many natural predators, you get to live in treehouses, etc. But...what if you could be a giant, superstrong, bear-man who could fix spaceships and everyone let you win board games? That would probably be a lot cooler. And it was almost the case.
In the original treatment for Return of the Jedi (REVENGE OF THE JEDI, I KNOW I KNOW), the Ewoks were Wookiees - folks from Chewbacca's tribe. First off, it would have made a LOT more sense for the rebels to want them to join their cause (since they're arm-tearing-off powerful beings) and it would have also made a LOT more sense for them (since their buddy Chewbacca was on the squad). And the battle against the Empire's forces would have been a lot more AWESOME BATTLE and a lot less "Home Alone"-style traps, since an army of Wookiees could do some serious damage to those Stormtroopers.
But being a helpless teddy bear has some advantages too, such as...your deaths are sadder?
4. Your Home Is About To Be Invaded By Darth Vader Admirers and Revenge-Seeking Imperials
Here's the thing - no one other than Luke knew that Darth Vader redeemed himself at the end of the film (well, Luke and the ghosts of Obi-Wan and Yoda and anyone they gossiped to). Any other evil Empire folks or Sith-wannabes would have thought Vader died true to the cause of the Dark Side. Which makes his burial place a spot they would flock to in droves.
There's a reason that the burial spots of many infamous evildoers throughout history have been hidden (Osama bin Laden buried at sea; Hitler was buried and re-buried multiple times, finally ending up in an unmarked secret grave that was also paved over; and NO ONE knows where the guy who invented Candy Corn is buried) - because they would become meccas for their followers. And now the Ewok village on Endor is the burial spot for Vader.
Odds are there were a couple of surviving Stormtroopers that sent word that they noticed Vader's body being burned on Endor. And once that happens, it won't be long til the Ewoks are eradicated - both to make room for the army of Sith-lovin' psychos that wanna set up camp near their beloved Vader's final resting spot, and for vengeful members of the Empire who want to punish the poor helpless forest critters for their part in the Empire's demise.