Why human beings generally ignore optional Activity

This is fact that human being is a selfish. he always do all things in his favour and ignore all optional activities which if he do later do problem in that ,'

If me take an example of young college engineering student , In beginning of semester they plan to work hard at the beginning,try to revise topic which they learn in college on that day ,but when they come to home or hostel they never do that. question arise that why this happens generally?

Answer is this that student postponed their schedule of student because the know that if they do it in later no problem arises. so they ignore that to study regularly

Now when exams comes they use a the quick review book for study, some time when there is a big gap between easy exams and small gap for tough exams they plan to study first tough subject and then study the subject whose exams is coming. they only plan but never implement and waste there time with friends and others to watch movies,exploring internet and in other activities they do that only because the know that this is an optional activity and if they do it later no problem arise and when the time of tough exam come they again pick a quick review books in which only important question and answers given by doing so they are just to pass the exam

So at last this is universal fact that the human beings always ignoring the optional thing/activities