The common cold and sinus infections are the two most likely causes of excessive phlegm, notes WebMD. A healthy body makes 1 to 1.5 liters of phlegm per day, and the majority of it trickles down the throat unnoticed. Excessive phlegm causes breathing difficulty, and the body demands its removal.
Phlegm or mucus is produced by tissue lining the mouth, nose, throat and lungs, according to WebMD. It traps unwanted bacteria and dust before it can enter the body. This is particularly important for sensitive airways. It also serves as a protective coating for the tissue, as it keeps it from drying out.