Why are some people so difficult?

Like someone said that they just don’t want you to just say hi and they need a reason on why should they talk to them or people don’t want people messaging them at all. One time I said hi to someone but they didn’t respond to me. What are they said is there a reason why you want to talk to me and I need to know why. That’s what I’m just want to be friendly. That person said not good enough and told me if I can’t get a reason don’t respond to them or I’ll block me. It’s not like I’m trying to be nasty.