It's highly rated because of its clever, if dangerous, design. The player is quickly drawn in to friendships with waifus who then take over their lives, really. Tasks are set and choices given, but these choices usually lead to dark avenues of depression, anxiety and psychological bullying.
It's a video story rather than a game, and so cuts a new avenue in computer addictions. The Japanese meaning of doki doki is the tick tick of a heart beat, so new players believe they are about to play a dating game.