Most of our choices are not rational decisions. Advertisers understand this very well. By appealing to our primitive and social agendas they try to trigger urges endemic to human nature.
Although milk may make us healthy (and therefore happy), it sells best when paired with someone beautiful. Advertisements influence purchasing habits by sparking automatic impulses. They are designed to trigger any number of cravings. This might be to attain something new, enjoy your family, have better sex, gain more power or prestige, you name it.