Disclaimer: I am not a psychiatrist and this is based solely on my anecdotal observation.

Susceptibility to depression seems to vary by individual not only in terms of severity but also with respect to type.

Three types I've observed in my family:

1. "Presence of" (or anxiety-driven) depression. This type of depression stems from feeling anxious about an impending (or potentially impending) event that you worry may be unpleasant, stressful, or embarrassing. In other words, it's the presence of something that's causing you to feel anxious, which, in turn, leads you to feel depressed (take away that thing and the depression goes away). A perfect example is the Sunday night blues. My mom and I mainly experience this type of depression.

2. "Absence of" depression. This type of depression stems from feelings of void or loss. For example, it often accompanies feelings of loneliness, boredom, regret, or purposelessness. It can also accompany conditions such as financial trouble (or loss) or death of a loved one. My dad mainly experiences this type of depression (especially since he retired).

3. "Unascertainable" depression. This is where you feel depressed but you can't pinpoint why. To me, this would be the most troubling kind of depression because you can't see a way out of it because you don't understand what's causing it in the first place. One of my sisters experiences this type of depression. It's a beast.

People generally seem to have more success using drug-free lifestyle interventions (e.g., physical exercise, meditation, etc.) for the first two types than the third type (I'm not a neurologist but there's obviously something going on brain chemistry wise with the third type that's much more difficult to address). For example, in my case, physical exercise does me a world of good. But it doesn't seem to work all that well for my above-mentioned sister (although she's recently reported having some success with meditation).

Another observation is that, between the first two types, introverts tend to be more susceptible to the first and extroverts tend to be more susceptible to the second (whereas the third type doesn't seem to correlate to either). In that regard, the first two types may be viewed as not being true depression but, rather, the natural response of a certain personality type to a given set of conditions.