Why do people on here get mad at you and blocked you because you don't get back to them in pm.real quick?

I had a big brother on here for a short while.and had to go to the doctor earlier.I forgot to signout.like I do alot of times and it says I am on line.but I am really not.so when I came back on.to answer him.he had blocked me.I wasn't ignoring him i wasn't online.I do forget about things that I was doing and do something else.I do have Adhd.and take medicine for it.so don't think I am being rude.or uncaring. If I don't get back to you right away because I am not. I just forget sometimes to signout.