I am curious now, what positive energy effect deals damage to the living?(not snarky!)
Alignment Channel and Elemental Channel allow you to harm living outsiders (of the designated alignment subtype or elemental subtype) with positive energy, or even to heal living outsiders with negative energy.
Exposure to the positive energy plane can also harm or kill living creatures.
In 3.5, the Ravid was a monster that harmed living creatures it struck by using positive energy. Just as any medicine, in high enough doses, is toxic, it seems that positive energy can be quite dangerous. (And, given that it encourages life and growth, it would seem an ideal way to cause tumorous growths or deformities, cause infestations and parasites to reproduce out of control in a living host, greatly accelerate the effects of disease, etc.)
There are also other effects out there that are "RAI" positive energy that damage the living, such as Holy weapon enhancement and the Searing Light spell. Lay on Hands wasn't listed in the CRB as a positive energy effect, and I'd like to think it only became Positive Energy in the FAQ because I pointed out it technically wasn't typed that way, some other Paladin powers like Smite could also be typed as Positive Energy, and they very much don't heal Dragons and Devils and such.