For me its when my family flew in for my graduation and they also decided to meet my sister's in-laws. After graduation we were supposed to go to a restaurant. The ceremony was supposed to last two hours but stretched to four and we were about to miss our reservation. My sister pressured me to leave even tho I didnt have a chance to take pictures with friends and such. I have mostly moved on but honestly a part of me still hates her for it.
For some small stuff: We recently held a big celebration for my wife and I's combined family (think 60ish people) to celebrate the birth of our son. At this event were a bunch of relatives that I rarely get to see, as they almost always decline any invitation I send them for almost any kind of event, so I thought it was a big deal to get to see them. However before I even had a chance to really talk to them, they just left without even saying goodbye or anything. They also haven't written to me or given any sort of reaction to what happened at that event, and I can definitely feel myself growing more bitter about them the more I think about them.
For the big stuff: I feel kind of bitter about the fact that my mother in law is a well-meaning fool, who keeps asking my wife for money every month, which my wife then obliges. My mother in law is in quite a lot of debt, and has been struggling to make ends meet ever since she got divorced more than 10-15 years ago, so I completely understand why my wife feels like she needs to help her mother keep a roof above her head. However, every time my wife and I get hit with a rent increase, we bemoan the fact that we are slowly having our savings eaten up with no apparent end in sight. We manage to make it through every month with a small surpluss each, but I definitely don't feel like there is any progress being made at all, which means I will be a perpetual renter for my entire life, as 1 income is flatly not enough to buy a house in this financialized shithole.