Do you make a living as a photographer?
Your brain is fine. It's probably that you're just shooting some easy stuff that allows you to take time to zoom, to move or even to change lenses. You'll know what I mean if you try to shoot with a 400mm lens cross the field while people running all over the place and you're shooting away at 10 frames per second. Hey I'll take the lucky few any day of the week.
Hmmm... If I have time to focus I sure have time to compose the image.You can't always capture it just right. This is especially true in case you're shooting sports, action or events that you really don't have a lot of time to stop and compose.
Maybe I have some sort of brain damage, but when I put my eye to the viewfinder I get into a zone were everything moves in slow motion... And time constraints are usually the least of my worries
(And yes , I am very selective before I release the shutter button - no way I am gonna shoot 100 images, hoping for a lucky few - fast paced action or not)