What Causes Flat Beer?

Compressed air or the improper mix of beer gas (25% CO2)

“Serving your beer with air is the equivalent to pouring a pint, setting it down, and then drinking it the next day.”

Flat beer is wasteful because you are putting extra (poor quality) beer in every glass. A 5/8" head is the equivalent of 4% of a pint. No head = 4% beer loss. Decreased Yield = decreased profit.
Air is one of beer's worst enemies; it begins to destroy the product immediately on contact.
Air contains oxygen, which will shorten the life of beer drastically; it can carry other flavours into the beer.
Air contributes to the growth of several organic contaminates, requiring more frequent line cleaning.
If the CO2 content in draft beer goes down, the beer will taste off and look unappealing (flat).
A 25% CO2 mix does not have enough CO2 for proper dispense of most non-nitrogenated (domestic) draught brands, causing beer to go flat.