A key to happiness that you must take from this article....

Happiness is not living a problem-free life. That is an unrealistic definition and by all means impossible. Because none of us will EVER be problem free in this inherently imperfect world. Happiness is learning to be content with yourself whilst persevering through the troubles you have, and to continuously focus on the other positive aspects of life.

I want to make it clear that I am NOT referring to clinical depression here. Clinical depression is more complex and severe and deserves professional attention. Clinical depression can have a number of causes ranging from neurological to hormonal to situational. The depression I am referring to in this post is the normal 'rainy day blues' that happens every now and again with all people. 

Just speaking out of honesty, whenever we have one of those days where we feel depressed but we can't pinpoint the reason, or maybe we can, but they're things that can be fixed. A lot of the time (but not all the time), all it takes is to challenge our own depressive thoughts in order for our blues to go away. For example:
'why do you feel like this?'
'why is this an issue?- is it really an issue?'
'okay, so it's an issue. but surely it can be fixed?'
'and if it can't be fixed easily, will it be worth it to make the effort to fix it?'
'If it cant be fixed at all, maybe I should learn to adapt?'

Either way, if you took the time to challenge these thoughts most of the time you will end up with answers along the lines of 'fix it and be happy' or 'adapt to it and be happy'. It won't be direct as I make it in the above example; of course mental endurance, patience & knowledge are needed to overcome a problem. But it is possible.

I notice that as people, we're very passive when it comes to attaining happiness. We're very much dominated by our environment. We underestimate our ability in changing our situations, or changing our attitude to adapt to a situation. We should be more assertive in our thinking and be aware of our own thought processes (meta-cognition). Really, speaking from experience, it does wonders in letting go of a lot of the emotional baggage.