I believe the reasons are simple.. either
1. Routine.. wake up, go work, go home, sleep? weekday too tired.
weekend do nothing. or simply work is too routine.. In this case,
break your routine.. try finding something to do in order to break
this cycle. Can be anything.
2. Fatigue. After you work for some years, you just like feeling a
break. Even the best job and environment which u like or super
interested in can get to you after some years. For this case, just
take a week or 2 leave and chill out somewhere for a while. After
that you be ok le.. at least for a while..
3. Interest. You simply have no interest for the job. And working
for the sake of working. Just think of $$$$$$~ you will need a lot
of that to keep u going~ 
