How does recalling a specific memory decrease depression? Research by Serrano et al (2004) indicated that 


  older adults with depressive symptoms showed that with increased
  specificity of memories, individuals show decreased depression and
  hopelessness and increased life satisfactions. - Wikipedia


Questions


Does the Seranno et al study  apply for most people?
How does recalling a specific memory decrease depression? 
Can it be any specific memory?


References


Serrano, J. P., Latorre, J. M., Gatz, M., &amp; Montanes, J. (2004). Life review therapy using autobiographical retrieval practice for older adults with depressive symptomatology. Psychology and Aging, 19(2), 272.
In a meta-anylysis, Bohlmeijer (2007) found that:


  In the last twenty years reminiscence has been applied in a large
  number of settings and with a large number of target groups. Examples
  of applications are: community-residents with a major depression,
  elderly with moderate depressive symptoms, nursing home residents,
  elderly with dementia, rural-dwelling older adults, elderly in
  assisted-living communities. ... We conducted a meta-analysis to
  assess the effects of reminiscence on depression across different
  modalities and target-groups. An over-all effect size of 0.84 (95% CI
  = 0.31 – 1.37) was found, indicating a statistically and clinically significant effect of reminiscence and life review on depressive
  symptomatology in elderly people. This effect is comparable to the
  effects commonly found for pharmacotherapy and psychological
  treatments. (p. 152f.)


To answer your first question, it seems that reminiscence and life review are an effective treatmtment of depression for a wide variety of (elderly) patients. It should be noted that a "significantly larger effect was found in studies in which life-review was the intervention (d=1.04) as compared to studies that used simple reminiscence (d=0.40)" (p. 152).

Several theories have been applied to try to explain how recalling memories decreases depression:


disengagement theory
ego-integrity theory
continuity theory
socio-emotional selectivity theory


Disengagement Theory


  The foundations for reminiscence and life-review therapy were laid by
  Butler (1963) in his famous paper The life-review: an interpretation
  of reminiscence in the aged. In this paper Butler put down his
  clinical observation of an increase of reminiscence – the act or
  process of recalling the past - in older people and postulated that
  this was due to the universal occurrence of an inner experience or
  mental process of reviewing one’s life. He conceived of life-review as
  a spontaneously or naturally occurring process that is ‘characterized
  by the progressive return to consciousness of past experiences, and,
  particularly, the resurgence of unresolved conflicts’ (Butler, 1963,
  pg 66). He hypothesized that it is caused by the ‘realization of
  approaching dissolution and death, and the inability to maintain one’s
  sense of personal invulnerability’ (Butler, 1963, pg 67). Though he
  recognized that people of all ages review their past from time to time
  and that any crisis may prompt life review, Butler stressed that
  life-review is more intensive and observed more frequently in (early)
  old age. He discerned adaptive and constructive manifestations of
  life-review from psychopathological manifestations. The adaptive
  variant is described as a reconsideration of former life-experiences
  and their meanings. It will often be accompanied by mild feelings of
  nostalgia or regret but is generally typified by expanded
  understanding and acceptance of one’s life, the experience of
  meaningfulness and declining death-anxiety. (Bohlmeijer, 2007, p.
  32)


Ego-Integrity Theory


  In its early stages reminiscence was studied mainly within the context
  of developmental stage theory (Erikson, 1963; Webster, 1999).
  Life-review was seen as a naturally occurring process which takes
  place in the last stage of the psychosocial development of human
  beings. The main hypotheses that could be deduced from this theory
  were that life-review is universal and old-age specific. These
  hypotheses were not confirmed in empirical studies. ... Based on
  studies rejecting the claim that life-review is old-age specific, it
  was suggested that reminiscence and life-review could be better
  understood within a life-span perspective (Webster &amp; Cappeliez, 1993;
  Webster, 1999). ...
  
  In addition to the disengagement theory (Baum &amp; Baum, 1980; Butler,
  1963) and the ego-integrity theory (Erikson, 1956; Taft &amp; Nehrke,
  1990), in recent years new theories are applied to reminiscence, for
  example the continuity theory and socio-emotional selectivity theory.
  These theories fit with a contextual and life-span approach to
  reminiscence. (Bohlmeijer, 2007, p. 37f., 40)


Continuity Theory


  According to continuity theory individuals, when confronted with
  life-events or transitions, ‘attempt to preserve and maintain existing
  internal and external structures and they prefer to accomplish
  this objective by using strategies tied to their past experiences of
  themselves’ (Atchley, 1989, pg 137). This sense of continuity, with
  the aid of reminiscence, will promote adaptation (Parker, 1999).
  Continuity theory would predict that people will reminisce more
  frequently during periods of personal transition than in more stable
  periods. In testing this hypothesis, Parker (1999) found that young
  people were significantly more likely to reminisce during transitional
  periods than older adults. (Bohlmeijer, 2007, p. 40)


Socio-Emotional Selectivity Theory


  Another theory applied to reminiscence is
  the socio-emotional selectivity theory (Carstensen, 1995). This theory
  proposes that with growing age emotion regulation becomes more
  important than information gain and that the elderly arrange their
  social life in such a way (concentrating on close relationships) that
  they can have as many emotionally meaningful interactions as possible
  (Pasupathi &amp; Carstensen, 2003). One important aspect of such
  self-regulation is known as the positivity effect (Carstensen &amp;
  Mikels, 2005), in which it is assumed that an emo- tionally gratifying
  focus ‘...would bias attention and memory in favour of material that
  optimizes emotion regulation (i.e., positive material) even if there
  are costs to focusing only on such material’ (p. 118). This hypothesis
  has been supported by many studies (Kennedy, Mather, &amp; Carstensen,
  2004).
  
  Older adults will therefore actively look for social
  interactions in which they reminisce because of the potential for
  emotion regulation and well-being (Penne- baker, 1997; Bluck &amp; Levine,
  1998). This preference for talking about the past in social
  interactions with strangers was confirmed in several studies
  (Pasupathi &amp; Carstensen, 2003). (Bohlmeijer, 2007, p. 41)


Note:

Bohlmeijer's dissertation gives a comprehensive overview of this area, if you want a quick introduction. It is available for free online (see link below). Serrano's article has been quoted 157 times, according to Google Scholar. Probably some of those articles will provide interesting criticism and more recent research not covered in Bohlmeijer's meta-analysis from five years ago. You can find these articles listed at Google Scholar.



Sources:


Bohlmeijer, E. (2007). Reminiscence and depression in later life (Dissertation). Am- sterdam: Faculty of Psychology and education, Vrije Universiteit. Available online at http://www.researchgate.net/publication/8958357_Effects_of_reminiscence_and_life_review_on_late-life_depression_a_meta-analysis/file/79e4151397c8674185.pdf