Question

When should you see a professional when your feeling sad?

Answer 1

If your friends or family notice that you're sad often than you definitely should.

Answer 2

You should see a professional If you feel hopeless and helpless, you've lost interest in things you used to enjoy, you feel tired all the time, your sleep and appetite have changed, or If you can't control negative thoughts and are irritable, short tempered or feel more aggressive than usual.
For all of these symptoms of depression you should go see a professional if the symptoms persist.