Am i depressed or just normally like this?

Hey, ive had a issue for about two to three years where im always tired and low on energy, especially after meals. I got my blood sugar checked and they said im fine, and they suggested that it might be depression thats my problem (im 15). At first i laughed at this notion, as i consider myself to be extremely stable minded, until i had a night recently where i seriously believed that i should give up on all my hopes and give my life to helping others. I came to the conclusion that some people who are born with great gifts and a kind heart are made to solely help others and ignore themselves, and my mom pointed out that i sounded extremely elitist in saying that im blessed with much greater attributes than others. Ive had a history of being a little self centered, but i started to think that maybe i am hiding a giant ego that is the source of my issues. I have recently become a very deep thinking and clever person, and ive started to develop internal systems of logic that i treat like my code for life, such as i never will do this and some things are naturally wrong. I noted that many people who thought too logically turned into tyrants when they justified their actions, so i incorperated emotion into my logic. Ultimately, my ideas seemed remarkably sound for my age, but when i started thinking too deeply, i questioned why i had the desire to live. The issue is, i couldnt find an answer. This is when i realized that for quite some time, i might have had moderate depression. Ive been rejecting social interaction some what for a while, and even though im good with other people, ive started to hate almost everyone. My question to you is, do you think im depressed, or just a naturally deep thinker, and if you have anything else to say.