• You can do it better yourself (perfectionism)
• You can do it quicker yourself (short-term view)
• Lack of confidence in co-workers (need to put in time to train others)

Reasons to delegate:

• You do not enjoy doing this activity.
• You are not good at doing this; someone else could do it more effectively.
• You can leverage your time better by handing this off to someone else, even if it's something you enjoy and/or are good at doing. (Just because you are capable of doing something well does not mean that you have to do it!)

Tips for effective delegation:

• Explain task specifically and thoroughly
  
• Define purpose. What are we trying to accomplish? Determine what success looks like so the person has a clear picture of what we are wanting to accomplish.
  
• Delegate responsibility, not work.  Too many managers confuse delegating responsibility with offloading work onto someone else.  When assigning a project, allow your employee the freedom to exercise some personal initiative.
  
• What's in it for the delegatee? Recognition, new skills, etc.
  
• If HOW it is done is important, give and get feedback about how to do the task.
  
• Don't hover. Show confidence in their ability to do the task. Give them freedom to do it their way.
  
• Instead of asking, “Do you understand?”, ask questions such as, “Any ideas as to how you’ll proceed?”  You’ll get a better sense of whether or not your request was clear.
  
• Make delegatee accountable. Establish deadlines.
  
• Keep a delegation log.  Click here for an example.
  
• Give positive and corrective feedback. (It looks like there's a mistake here. What do WE need to do to get back on track?)
  
• Give recognition.
  
• If you can't delegate something, you may be able to renegotiate expectations.

For a more information on this topic read

The Fine Art of Delegation.