A person who obsessively indulges in social issues to stimulate strong internal emotions for their drug-like effect. This can be an unconscious behavior; the emotion junkie will often be able to rationalize support for their cause/drug of choice.
What separates emotion junkies from normal activists is a distinct lack of empathy towards other nearly identical causes, and flitting from cause to cause as the emotional intensity of each waxes and wanes.
Emotion junkies are unable to give a coherent rational explanation for their obsession with any given cause. However not all people who lack rational understanding of what they're doing are emotion junkies.
Animal rights groups are fertile ground for emotion junkies. Members of such groups who live to emote, but are disinterested in strategy or other organizational aspects tend to be emotion junkies.
People who are suddenly obsessed with certain celebrities, particularly after they've died. For example a person who spends vast amounts of time and energy with tributes to deal celebrities, and are active in rumor mongering myths about the circumstances of their deaths.
People who obsess over one and only one egregious crime reported in the news, but pay no attention at all to other, equally egregious crimes in the news.