We use terms like depression as a bite-sized catch-all to help usefully explain experiences and feelings to ourselves and others around us, to maximise understanding and empathy and to create realistic goals for managing and curing mental disease that can be replicated across the board.
Depression is, first and foremost, a emotional and attitudinal problem, characterised by a lack of interest and enjoyment in otherwise enjoyable and engaging activities, suicidal thoughts/ideation, and a feeling of hopelessness. I never had all of the symptoms of depression, and after many years of learning to manage it, I found that I lacked pretty much all the symptoms apart from a few core ones. Similarly, I say I have depression but I do not feel the symptoms all the time.
All of this is because mental illness is not as clear-cut as we would perhaps want it to be. The check-list of depression is a list to aid understanding and to help overworked doctors make quick diagnoses so you can be helped sooner. It's also there to aid you in feeling more 'legitimate' in your own illness - bear in mind that people who meet every criteria in the list can and do feel that they're secretly faking it and are also being pathetic.
This checklist, of course, have it's drawbacks. If you know you feel different than you used to feel, even if it isn't the classic, popular view of 'depression', then all that matters *you* know something is wrong, and you are welcome to categorise that experience as a problem, and you are welcome to seek help for it.
We as a society are gradually beginning to understand that mental illness is not simply a matter of internal turmoil. It is greatly affected by, and perhaps even caused by, the environment around us. Mental illnesses can have difference prevelancy rates in different cultures, for example. So, there is no reason to think of yourself as pathetic, regardless of how severe your depression may be.
(Also, for the record, I've never had the symptoms of disturbed sleep or loss of appetite either, and I'm diagnosed with depression. So just in case you didn't want to read all of that, you can read this bit and feel assured).