Depression is cruel and evil mistress that sometimes seem as no one can escape from her clutches. It's not something that I would wish on my worst enemy. What causes is chemical disbalance in brain, but what triggers depression on each individual is different.
We sometimes view happiness as easy and depression as something that is self indulgent, when it's an in fact a mental battle where you feel you are drowning and yet you are ashamed to ask for help, if you feel like drowning its not something to be ashamed of. Talk to anyone, suicide is an long term solution to short term problem,don't struggle alone.
Here are few sites to check
Samaritans 08457 90 90 90