
Sounds more like the St Bernard is giving off some kind of scent which is, erm, "arousing" for your rottie! I assume the SB isn't on any kind of hormone treatment for an unusual condition, or perhaps on other medication which could make his scent different maybe?
What about something used in the SB's household which the rottie could be fixating on - the owners perfume/aftershave? Certain pheromones can have this effect I believe. What about any cleaning substances used on the SB him before a show? Shampoos, colognes, etc?
I don't have any experience of this, just thinking aloud really! Of course, it could simply be that your dog finds the SB devastatingly attractive. :)
Sorry I couldn't be of more help, but I'm sure somebody else will be along who can shed a bit more light!
Good luck
:)