Why does my kid walk on his toes constantly?

Possiblities: Tight tendons, muscle weakness or even neurological conditions. See pediatrician and podiatrist who specializes in childrens conditions.

? age of kid ?: Toe walking is a common habit of some young kids that usually resolves by two or three. If it persists & it may result in gradual shortening of the tendon attached to the back of the heal. Monitoring by your pediatrician may include some heal stretching exercises & physical therapy. A visit to a pedi orthopedist may be needed.