Let me put it in the simplest way possible, as I view it.
Many activities can be considered pleasurable to people... Buying stuff, feeling part of community, posting on forums, eating food they like. For people within "normal ranges" they find a balance of happiness and stress or somehow just push on.
Medically though, unipolar depression and bipolar depression are quite unique cases where chemical imbalances, differences in brain structure etc. can be observed, on top of behavioural and emotional symptoms. With depression, it's not like you feel things are "sucky", it's as though the "bottom has dropped out from under you", and this feeling persists for days or weeks at a stretch. Suicidal ideation, even if philosophically considered in "normal" states, takes on an edge of more planning than usual. "What if I..." etc.
Without medication or psychological counselling, anything can help alleviate some symptoms of depression... Such as excercise, love, friends, sex, Apple products, Android products, a hike in the park, lectures, music, work, study, books, eating and so on. Recreational drugs can boost feelings but have short-term and/or unpredictable longer-term effects.
For sustainability and stability though I think medication with appropriate psychiatry and psychology form a good base from which you can then apply those things that are pleasurable.
The analogy is high blood pressure, diabetes and cholesterol. Yes, lifestyle, diet and herbs can all help but in combination with suitable medication you get the best outcome.
I'll get off my soap box soon but mental illness is not a moral weakness, nor an excess of emotions. It is a distinct and treatable medical condition. Unipolar depression can be curable if there are one or only few occurrences and a later age of onset. Bipolar is treatable but not curable at this stage.