Economists who consider trade deficits to be bad believe that a nation that consistently runs a current account deficit is borrowing from abroad or selling off capital assets—long-term assets—to finance current purchases of goods and services. They believe that continual borrowing is not a viable long-term strategy, and that selling long-term assets to finance current consumption undermines future production. (If this reminds you of the discussion about federal budget deficits and the national debt, that's no accident the mechanisms at work are similar.)
Some economists who oppose trade deficits see them as a symptom, rather than a cause, of trouble, specifically bad central bank policy. They believe that trade deficits arise from loose monetary policy. A rapidly growing money supply boosts demand, including demand for imports.
Economists who consider trade deficits good associate them with positive economic developments, specifically, higher levels of income, consumer confidence, and investment. They argue that trade deficits enable the United States to import capital to finance investment in productive capacity.