The Cause: Not Enough Light
This is almost always the reason. The plant is stretching to find more light. The stems get long and the leaves are spaced far apart. The plant is putting all its energy into reaching a light source.
The Fix
The solution is twofold. First, move the plant to a brighter location. Second, you can prune the leggy stems back. This will encourage the plant to branch out and become bushier and fuller. For vining plants like Pothos, you can take those cuttings and propagate them!
