You have probably noticed that it is easier to sing while standing rather than sitting down. For some reason, your voice does not feel supported while sitting down.
Why is That?
When we are sitting down, our posture is likely to be out of alignment. A good posture for singing is a straight line running down your ear, shoulder, hip, knee and ankle (when standing). While sitting, aim for a straight line from ear, shoulder to hip. So, if that straight line is broken, your posture is bad.