Something alone the lines of "I'm very sorry, I have an allergy to dogs. If this one remains as close to me as it is now, I will be extremely ill, and I wouldn't want to put you to the inconvenience of taking care of me when that happens. I wonder if you'd mind very much asking the passenger in 1F to move to another seat further away from me?"

On both occasions, the FA has deftly managed to get passengers to trade seats so that I could be accommodated, and the dog ended up in 3F. I think the fact that I boarded the plane before the other guy arrived was helpful. I don't know what would happen if I arrived after the other guy had already taken his seat. Another reason for being one of the gate lice!