Humans can only focus on one thing at a time. Nobody is really sure why this is the case, but it is true that if you pay attention to one thing, the stimulation from the focal object seems to shut out all other stimuli. This is both a blessing and a curse.
Yes, there are many people who would like to be able to multitask far better than they are currently able, but the payoff might not be worth the sacrifice.
Many studies have shown that multitasking increases stress as the amount of perceived stressors goes up. It also slows the mind down and makes it less effective. Dividing and distracting the mind causes mental course changes and thus stagnation.
Think of a dog who is trying to pursue three different balls thrown in opposite directions. She runs two steps in one direction, three in another, and one step in another. She ends up not going anywhere at all. The mind is the same way.
The more time you spend changing mental direction the less time you have to actually focus on the task at hand. The more directions or mental tasks you add, the less effective at them you become.
But that's not the whole story. When worrying about events or tasks that are not directly involved with the present task, you are in effect worrying about things that you have no control over. In that case, worrying about it will not make it better, and will only tax your mental and emotional resources, making you less effective.
You cannot gain anything from it, only lose, and if there is a fight you cannot win no matter how hard you try, then it is probably best not to engage in the fight in the first place.
Others might argue that if they can't use their conscious mind to multitask, then why not use the subconscious instead? While it may not be obvious, people use their subconscious mind a lot more than they think. It is actually one of the most powerful tools humans possess.
When answering a question about what happened last week for example, a person doesn't usually have to consciously go back and examine every minute of every day to find the answer. Instead, the subconscious mind takes over, instantly searching the brain's archives, finding the correct answer, and then delivering it to the conscious brain.
This is typical of almost any task involving memorized actions and facts. Like conscious thoughts, these subconscious processes also utilize brain processing capacity and energy, and even take a toll on your emotions.
When compared to the parts of a computer, the conscious mind is somewhat like the user interface, the part that becomes visible on screen, which can then be affected by the click of a mouse to execute a command. The subconscious on the other hand is like the processor.
While invisible, it actually does most of the work, executing on every command within milliseconds after the user initiates it. Like a computer though, if you ask it to run to many commands simultaneously, or a command that is far too large and complex, it will freeze and lock up.
For example, let's you are trying to finish up an urgent project at work, but you just recently had a fight with your partner. Your subconscious mind has not stopped ruminating on what was said and felt during the argument, and you suddenly find yourself far less able to focus on the project, and you are much more irritable than usual, even though you are no longer actively thinking about the fight itself.
This happens because while your conscious mind is focused on the project, your subconscious continues to reflect on it, and react emotionally to it. The emotional response of your brain then has a physiological and psychological effect, absorbing all of your processing capacity and coloring every other thought you have, even those that have nothing to do with the fight at all.
Another common objection to the philosophy of living in the moment is that the present isn't the only important time frame. The future is important too. While that is definitely true, there is a glaring caveat that needs to be added to that phrase.
While we should think about the future, even the distant future, it is something we should do only occasionally to redirect and orient our current actions towards where we want to go.
It should never be something that consumes a large percentage of our mental time, because like the past, the future cannot be changed by thinking about it. It can be changed however by what we do in the moment.
Stressing over the future is dangerous for another reason, as it allows you to effectively stack up multiple stressful "moments" at once. This can overload the mind, causing an emotional crash (going back to the computer analogy) followed by mental paralysis.
The brain does not always do a great job at separating ideas out through time. It tends to only look at the absolute size of the task involved. So if you can break your tasks and challenges down into bite-sized chunks, your mind will not perceive as much stress, and will not balk at the task so easily. This is a great way to cure a stubborn case of procrastination.
Those are all the negative aspects of focusing on too many things at once, but there is an upside to this phenomenon as well.