shipping obsession is in conjunction to people's love and romance and relationship obsession in real life. in real life, a lot more people gossip and care about others' love-lives than exactly how we will win some war or something, 'cause mostly people know there's nothing they can contribute or change about said war, but in terms of other people, we all have some sort of power, some sort of affective capability.
it's not a simple easy/hard sort of thing, it's a basic skill level sort of thing. relationships are something almost everyone understands, while plot-arcs and even character-development arcs take a greater amount of insight and intelligence to have coherent opinions on.
as far as why should people be so hung up on love-- well, that's understandable. unlike politics and things, love/sex is one of the basic human drives, along with food and shelter and um... breathing, i guess. it's an instinct. it's quite natural to find it more important than some grander aspect of "plot", which people respond passively to. like, in real life-- a lot of people respond passively to big events-- they -deal- with them, try to minimize damage, but most people don't have the capacity to be pro-active and preventative. it's not that it's -hard-, it's just... well... it would take more effort and thought (as in, -any- effort and thought) than your average reader wants to put into recreational reading~:)
( depends what you mean by `plot' and `fun' of course, so um... mileage may vary -.- )