Because when you multi-task, you are able to fool your brain that you are fulfilling much more work in single time and your brain rewards you more by increasing the release of one neuro-chemical (called dopamine) which makes you feel happy. Going further, let us tell you that multi-tasking is worst-tasking because in disguise of multi-tasking, you are just switching between tasks & task-switching experiments suggest that time cost of task-switching is much more than the time saved by multi-tasking. So, Be Smarter! Finish one task before switching to next one. Big No to multi-tasking.