It's not an anger thing. I'm not mad when people do it. But it's a time wasting thing and I'm not gonna waste my already under-available time. This gets pushed a lot within my work. Senior devs get a lot of messages. I regularly am spending a substantial amount of my day dealing with messages asking for help, reviews, and more, so anything they can do to be more actionable makes things go better for everyone.

Also, there's some people that take "hi" messages to extremes, as they won't even send their actual message until you reply to the "hi".