After some research, I was elated to find out that studies have been conducted, about dogs in general and specifically about their affection towards owners. As a disclaimer, I don't claim my answer proves dogs can love their owners - but existing evidence seems to suggest they do.
In order to determine whether dogs can posses deep affection towards their owners, we need to establish whether dogs have such feelings. From this study:
The results suggest that playful,
social, exploratory, avoidant and
aggressive behaviour in dogs is
influenced by stable dispositions;
i.e. personality traits, that seem to
have been important during the
evolution of the domestic dog.
It's behind a pay wall, so unfortunately we cannot scrutinize the methodology, but this study concludes that dogs do have what could be classified as personality traits. If we accept this, then we can safely assume a dog's behavior - more specifically, behaviour identified with affection - can very well be influenced by factors outside the physiological realm.
Then we have this study - it's research subjects are specifically dogs living at rescue centers, so whether it's conclusions can be applied to dogs in general is debatable. However, citing from the abstract:
The specific response of the handled
dogs toward the handler fulfilled the
operational criteria of attachment. In
shelter conditions, the remarkable
demand for social contact with humans
may result in rather fast forming of
attachment even in adult dogs.
In conclusion, existing research seems to corroborate the common sense claims that dogs do develop affection towards their owners. As for how much affection, I doubt research is available. However, anecdotal reports abound, such as the very touching case of Cpl. Liam Tasker and his dog, Theo:
The dog died shortly after it's owner was killed in a firefight, for no apparent reason other than the death of it's caring owner.