Why do people grow weary and lose heart???

I was thinking about this today and thought I would jot down some thoughts.  The Bible says don't grow weary in well doing because in do season you will reap a harvest.  So why do people grow weary.  I believe that one of the main reasons is they just go through the mechanics of Christianity or their faith.

When I go through the mechanics in my faith I start to lose heart.  

When I go through the mechanics in my marriage, my wife and I start to just get use to each other, instead of appreciating each other and keeping the flame alive.

When I go through the mechanics in my job I lose passion for getting up and going to work to make a difference.

Have you lost your passion?  Passion for God?  Passion for work?  Passion for parenting?  If so maybe you are just going through the motions and need to ask God to renew your spirit.