Why do Muscles feel tight?

Why do muscles feel tight? Is it because they can't relax? What should I do about it?

According to Better Movement by Todd Hargrove, tightness is a feeling, not just a mechanical condition. It's not necessarily a physical condition of shortness that needs an aggressive structural solution. You feel it more when you are sensitive and it will go down if you improve your overall fitness, strength, awareness, motor control, and health.