For coevolution to be coevolution (and not just two species hanging out whilst they evolve) the species have to influence each other's evolutionary journeys.
But rather than bore you with definitions and examples of what isn't coevolution, I'll give you an example of what is. The best example I know: bees and flowers.
But before I get into that, here's a rare glimpse into my creative process:
And, since dinosaurs went extinct 65 million years ago, that means they never got to see the really cool flowers. If that's not depressing, I don't know what is.
So back to the actual subject of this post, which is definitely not sad brontosauruses.
Flowering plants have evolved the brightly colored, lovely-smelling flowers that we happen to like because these traits attract bees. The flowers that attracted the most bees were more successful reproductively because, as we all learned in 1st grade or something, bees assist flowers by distributing their pollen. In other words, they act as a plant sex mediator.
Some flowers have evolved to actually resemble bees. A bee then comes along with the hopes of reproducing with its own species, only to discover he is actually playing into the flower's reproductive plan. The poor, confused, sexually frustrated bee is just the flower's pawn.
You can see why the bee might be confused.
So if it weren't for bees and their flower-pollinating behaviors, there wouldn't be any pretty flowers for us to look at. Thank you, bees, but please stay away from me because I don't want to get stung again.