This film works because...
...you genuinely like the couple. Too often in genre films today there is one of the small group of characters that is a total jerk. Typically, in this film, it would have been the husband. Had that been so, the unfolding of events would have not been so heart breaking as well as creepy. So cheers to the writer and director for steering clear of that modern stereotype.
In another films forum where there were "no likable characters" I proposed that the trend, especially in found footage "reality" films, is that perhaps it is a mirror of our society where there are few likable people? Manners, for example, have gone out the window to the point where people today do not even know they are being rude! So maybe we're all jerks and a good number of films simply reflect that? Loving couples as depicted here are the anomaly.