I feel that social species form intense bonds with one another, and since humans and horses are both social species, it's possible for us to form tight and lasting bonds with one another.
I believe the issues come when the human half of the equation tries to anthropomorphize that connection, or apply motives where there are none. A horse is still a horse, with horse motivations and sensory input and memory. We're their stewards, guardians, leaders and teachers, first and foremost and above all. We do it because we love them and it is what is right, not because we expect them to love us back for it.
To me, horses are a curious mix. Part pet, part livestock, part child, part dance partner or working partner. Part of us loving them is caring for them, and respecting what they are by nature.