Your description reminds me of The Vanished by John Peel... there are a number of differences, though, so it may not be the book you're looking for.
The book does begin with a teen who wakes up to find all other people vanished, the streets and cities empty, no one there. He drives around, and ends up finding a few other people - maybe four kids total remain, two boys and two girls? They do stick together, hole up somewhere - can't recall if its a mall, but it'd make sense - ans spend time trying to figure out what happened as well as figuring out how to survive with no one else. And in the end it does come down to two, one boy and one girl (and yes to romance), on a new and empty world that will belong to them.
So, like I said, quite a bit of similarity.
The differences are, that, in this book there's a subplot in the "where did everyone go" category, the kids hunt down answers and eventually find someone who knows (not a teen, an adult in an environment suit), and find out what happened.
what actually happened, was, the kids were kidnapped away as part of an experiment on communal belief creating reality (people who reinforced, or enforced this belief, kept it stable when individual belief might fluctuate, were called cornerstones)... the world they were on was actually empty and uninhabitable (toxic) but because they didn't know, they believed they were still home and created or superimposed their expected, habitable, surroundings on the new world. It was thus necessary in their eyes for the kids to be kidnapped, and then to be kept ignorant, since doubt would undermine the habitable... illusion? terraforming? whatever. And the kids would then die.
A plot point ended up being the empty cities, with all the technology, etc of their world was too complex, and their numbers (~4) too few, to sustain the terraforming. I don't recall what happened to the other teens - captured, lost, dead, decided to cooperate with the ones running the experiment... The last two end up running away, and decide the empty pastoral garden is much more sustainable - so it ends up being their own personal garden of eden, or somesuch.
So while there's a great deal of overlap, the missing where-did-people-go plot point is pretty significant to the novel, and if it wasn't in your recollection this may not be the book you read. It seems close enough to offer, though.