Mood Congruent Memory Bias
Mood congruent memory bias is the tendency to more easily remember events that have a congruence with one's current mood.
For instance, it is a holiday and you are feeling happy and relaxed. That mood by itself can evoke other memories of holidays, fun times, family get-togethers, etc... Anything that shares the current mood.
Likewise, something bad happens in your life. This brings up similar bad memories of negative events and can plunge a person into a spiral of despair. Although the actual process behind this is not well understood, it is very common.