Why can't I concentrate on the task I've been given instead of being distracted by everything else?

That's because we're focused on the wrong thing. To get a longer attention span — even a span long enough to read this paragraph — don't worry about managing the information. Worry about managing your attention. Paying attention, for long periods of time, is a form of endurance athleticism. Like running a marathon, it requires practice and training to get the most out of it. It is as much Twitter's fault that you have a short attention span as it is your closet's fault it doesn't have any running shoes in it. If you want the ability to focus on things for a long period of time, you need attention fitness.

Because maybe the task given to you isn't something that you like. Or maybe the distractions are really good but whatnot.. Just make those distractions into inspirations (: find beauty in them to finish the task that you are given.

Maybe everything else is more important than the task? Do not blame yourself, ask: Do I need this? Is this task really important for me? Can I solve the problem? Do I have the knowledge? Maybe, something of utter importance is on your mind that keeps you busy while you are on your task? What is going on in your life that needs solving, consideration or attention? Maybe, you have to learn how to focus? Try to do bow shooting, chess, or a computer game that asks for long periods of attention. Another thing would be meditation. What ever it is, you have to be honest to your-self and do not try to evade the truth. Lawrence

because i don't like the task, or the task is not my interest subject, or my head is occupied with something else

Maybe there are bigger things at play here? You need to ask yourself this question - why are you putting off doing what you think you should be doing? Once you realise that, you can work towards eliminating the distractions and completing your tasks.

Because there are other things that are taking up brain space because they are unsolved issues that are stressing you out. Or, the task at hand is something that is of less importance.

It could be ADHD just a hyper active mind, but it could be you have no interest in what you've been given, this could be a sign of anxiety or on coming depression along with or irritability, but more information would need to be provided to know for sure.

Focus is the key to concentration. Try your best to avoid or eliminate any distractions that disturbs your focus. If you have no choice and you can't do so, then you will just have to find ways to focus and not allow the distractions to distract you.

Perhaps you are just feeling stressed out about too many things going on. Try your best to concentrate on the task at hand. If you feel that medications are warranted maybe you can speak with your doctor about the possibilities of ADD/ADHD medications. They are very helpful for some people who have trouble concentrating on tasks. Best of luck!

There are a lot of things that can cause stress and distraction. People tend to be less distracted in areas they specialize in, or areas where they feel comfortable doing, but that's usually not the case with work. It's okay to be distracted, but you need to find a special time to be distracted. Make a goal for yourself, small steps to do before you relax for a bit, usually it's 30 minutes work and 5 minutes break for me. It helps to get your concentration back!

You can start by disconnecting everything else than your current task. Give your task just 25 minutes of pure attention and tell yourself that you'll get distracted only after these 25 minutes. This is a classic technique called pomodoro which increases your attention span. It has worked for me quite a lot in recent past. Slowly and steadily you can increase the time limit as well. Just start from 15 minutes and increase by 5 minutes per day. You'll see tremendous results in just a month of doing your work in such a manner. This way you'll get more things done and hence will be more efficient.