- The past keeps coming up
The happenings from your past can place a negative mood without your consent. If you've not fully understand how to handle the feelings from past wounds, then you're vulnerable to depression and sadness once you remember them.
For example, if a partner in a past relationship betrayed your trust, similar behavior by a new partner can trigger the same painful emotions, sometimes without you realizing where those emotions are coming from. The subconscious mind remembers everything.
In the midst of this painful experience in the past, these negative memories tends to act like an undercurrent of painful thoughts that has the potency to influence your moods.