Hedonism. Some people feel they have the right to always choose more pleasurable over less pleasurable experiences. Solution: Realize that your life's worth is what you've produced. That's true whether you're a dishwasher or a doctor.That often requires choosing less pleasant tasks.
Fear of failure: Many people know, from previous experience, they'll find tasks odious and will often do them poorly. Solutions: Get a less demanding job, exchange tasks with a coworker, get help on projects, get training, avoid perfectionism--especially on first drafts.
Dependency: Whether you're a trust fund baby, welfare or unemployment recipient, stay-at-home-parent, or have parents who'll always bail you out, dependency can demotivate you. Solutions: Fend for yourself--give away your money if necessary. And remind yourself that you don't want to be a parasite on this earth: you are worthy to the extent only that you've been productive, having had a positive impact on those around you globally and/or locally.
External locus of control: You place too much responsibility for your success on external forces, for example, God, racism, or spirituality ("the universe will provide" and similar nonsense.) Solution: Yes, not everything is in your control--luck plays a role--but success lies mainly within you, sometimes with assistance from others you trust.
Lack of drive: Some people are lucky enough to be born with high drive, others not. Solutions: If you're not, it may help to find a partner or a cause you deeply believe in. Sometimes, having a spouse and/or a child can add drive.