What is the reason that some people are annoyed very easily?

like just they get annoyed about just anything, what is reason? why some are opposite?
Update: I might be like that a lot of times . Like my neighbor was using this edging tool last weekend, and I understand it is his property and his time off from work, but it as irritating the hell out of me for some reason. especially since it was near dark .
Update 2: & I dislike people just walking in on me while I am in , for example the kitchen, or just anywhere , another example outside, the neighbor walks over and says something to me , chit chat, etc. , I hate how some just sneak up on you and then you have to say something to them or you look unfriendly or hostile, etc.
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