Normal is a setting on the washing machine. Not an accurate description of people.

Part of the definition of a disorder is that it causes problems for you or for other people. Enjoying solitude does not constitute depression, but that is just one thing and you have to look at the whole picture. Isolation implies a lack of supportive relationships and tends to move more in the direction of being a problem. Lack of motivation even more so, and that you associate motivation with disappointment sounds like there is something there that is bothering you and interfering with doing things that may be important for your life and your future. People are going to be concerned about that. You might ask your pdoc and others who know you what concerns they have, and then try to listen to what they say without arguing with them. Really try to understand why they think you have a problem.

It is fine to be an individual and to embrace your differences as long as they are not things that put you at risk or threaten to harm someone else. There is nothing wrong with being an introvert. It is how these things affect you that counts.