First off i dont mean to offend, but this will be the 3rd directed help i've supplied on this thread. Right depression (i've never been diagnosed with it), as much as i'd like to avoid a prognosis of myself it is necassary here: between age 12 to 15 years old i was depressed, by the end i had significant acne, 32 fillings, and i hadn't washed for 18 months (i stuck one of my books solid with dandruff), i had broken up with my best friend, been humilated and exposed for fancing a girl for 3 years. The fact though it was never address thus diagnosed, so u can probably use against me if this is misunderstood.
Depression is the absence of positivity, in a scenario when one is isolated one tends to favour negativity. You only need one person to be positive around you for you to latch on. Thus one could argue that having a positive individual around you is key, but in time you will learn that this positive person can in fact be you. It is this realisation which puts things into perspective.
To my knowledge i've not been depressed since 15, but then i could be in denial . I live with the words "there's always someone worse off than you" as a guide also.