I suppose it's contextual, to a baseline. Maybe one doesn't feel "motivated" to work on their startup idea, after 40 hours of conventional work. Maybe that's your body's way of telling you, you've had enough, and you shouldn't over-exert yourself. But if you weren't working at all, it would be a different conversation.
I bet when one notices a lack of motivation, it's a lack of motivation in a certain arena, not motivation in general, a lack of motivation to what one think they SHOULD be doing.
If you read about the symptoms and recognize more than one in yourself, and you still aren't able to commit to seeing a health care professional, confide in a friend or partner and ask them to help you through the process and hold you accountable, because depression is like a parasite that won't let itself be treated.