Maddy,

Situations may lead to depression.  The term situtaional depression gets thrown around alot but when you get down to to it depression resides within the individual and is a combination of biological, psychological, and behavioral processes.  Exposed to the same negative situation, a group of individual can have varying reactions.  Some may  become severely depressed when others show few or no symptoms of depression.

The way a situation is processed can lead to an imbalance in neurotransmitter levels, just how one thinks about the event.  If you are genetically predisposed to depression, psychological and behavioral responses to a situation can trigger the biological process.  The thoughts and reactions to the situation and depression continue to worsen and perpetuate the depression.

The best course of action, if you know you are predisposed to depression is to see your psychiatrist to add or adjust medication, treating the biological component, and to see a therapist to address the psychological and behavioral patterns that may be contributing to your depression.

There will always be shitty situations.  How we think about and react to them can be improved.  Knowing and recognizing the initial symptoms of your depression, as an indicator to seek medical intervention, being aware of the consequences of not getting treatment and working with a therapist can help combat depression and get you through those shitty situations much easier.