Well, I drive my wife crazy with this one all the time. Even when she starts a statement with, "You don't have to do it right now...", or, "Finish what you're doing first..."
Seldom works.
Why?
There actually are a couple of reasons...at least in my world.
WHAT DOES THE "H" STAND FOR IN ADHD?
Hyperactivity, right?
In a kid, you expect a lot of running around, but, in some adults, at least, "hyperactivity" can simply be the Mad Hatter in your head telling you to "Move along...."
As soon as you add a new item to my "list" of things to do, my brain shoves it to the top, and I feel almost compelled to start on it. Right away!
Another reason, at least for me, is years of experience in the fact that, if I don't do it now, it's going to get lost in the sequence. I may have the best intentions in the world of doing "B", as soon as I am done with "A", but, if "C" comes along in the meantime, I am going to forget all about "B". In fact, I might STILL quit "A" before I am finished and move on to "C"...never mind about "B".
In other words, if I don't do it now, it probably isn't going to get done. Then, I will feel bad at another failure to do what I was supposed to do, and, whoever wanted me to do whatever is probably going to interpret my failure to follow through as laziness, or that I just don't care.
Of course, when I drop what I am doing to do what I was asked to do, I am also failing to complete that task.
If I was doing something for someone else, now they are going to be disappointed. If I was doing something for the person who requested "B", they will not be happy that I completed "B" (or at least started on it), they will probably only be upset that I didn't finish "A".
If I was doing something for myself, I will see not completing my project as a(nother) failure, AND, I will be upset that I was not allowed to complete what I wanted to do.