could be, but definitely a oversimplification - maybe even to the point of being flat out wrong. we don't really have a good understanding of depression yet. the monoamine hypothesis (depression = too low levels of monoamines, especially serotonin) was the most popular in the last decades, but it has serious flaws, that cannot be overlooked. for instance that ssris don't help nearly as good as would be expected (barely above placebo for most drugs), that ssris aren't effective against depression (if they are at all for a particular person) from the first dose or that some antidepressants like iprindol have no effect at all on serotonin levels at all but are just as effective as common antidepressants. there is some kind of association between lowered serotonin and depression, but postulating a strict causality is surely wrong.