It's human nature to get upset over something. What truly matters is understanding what is really upsetting you and being able to rationalize whether it's worth spending the energy on. If you were to lay everything that upsets you out on a table in front of you and categorize them, you would most likely discover a pattern leading to the root of your frustrations: your expectations. It's perfectly logical. We become upset when what we anticipate to happen doesn't.
With every person we meet and every situation we encounter, we instinctively create expectations for how someone should behave or how a situation should play out. However, things seldom go to plan and people will, more than likely, not act as you want them to. This being said, the more expectations you have, the more chances you give for people to disappoint you. People who hold too many strict expectations are the most likely to be let down and live miserably. You cannot control the world around you, you only ever have control over yourself and how you approach things. The sooner you accept this and relinquish your desire for control over your surroundings, the sooner you'll find yourself living more happy and carefree.
Living life with little to no expectations reduces disappointment exponentially as you have nothing to compare people or circumstances to. Having too many expectations is like carrying a large stack of books. Every time you let go of an expectation you lessen your load and are able to see the world around you more clearly with much less of a burden. When you expect nothing you are also able to be more appreciative of what life hands you. How can you focus on the blessings you receive if your hands are already full with expectations that aren't even likely to be met?
The key to impactful growth and lowering your expectations is honesty. Speaking from personal experience, I used to believe I was being brutally honest with myself and yet I'd still find myself making excuses for my actions and brushing them off rather than getting to the root of the problem and solving it from there. I did this for years and, as a result, found myself drowning in excuses upon excuses. I was lost in what was real and what I had told myself was real. If I had just been truly honest with myself I could've avoided so much pain that I not only inflicted on myself but others as well. Lying to yourself and hiding behind meaningless excuses only really hurts yourself at the end of the day. Being able to understand what's actually upsetting you is absolutely necessary if you want to solve your problems and transcend them.
No one likes being disappointed. Rather than falsely attributing the fault of something to external influences, try changing your approach to it instead. I've said this before and I will continue to say it. Life is all about perspective and the choices you make. Choose happiness by expecting nothing and learning to appreciate everything.