
I've had leonbergers for 25 years and sounds like you have been given a bit if a rose tinted glasses picture or a good sales talk


They are obviously a fabulous breed but especially young Leos can be very far from gentle calm giant! More so a bull in a China shop

They can be pretty energetic, especially teenage boys can be hard work to guide them to become fabulous adults...
Facebook has a great group called -so you want a leo- realities of living with a giant dog...
I have 6 at the moment ( between 10.5 years and 1.5 years old) but you do have to be special kind of person to have a Leo & enjoy living with all that comes with them :-)
I have many puppy families where Leo has been a first dog and they have done great - gone in the ownership with eyes open and ready to train and work with their dog. I do tend to select easier pups for these families so they can cope and will not be walked all over by their bundle of fluff

No breeder should be telling leonbergers are not aggressive. Any dog can be aggressive and there most definitely are leonbergers that are not great with other dogs, some males do not tolerate other males etc normal dog behaviour can be expressed by leonbergers too, which can bring it's challenges when dog weights 10+ stones. Granted, they are generally pretty easy going but they are not always gentle all tolerating teddy bears.
Many also have a bit of a guarding instinct, not of a level of many working breeds but let's say I do not have to lock my doors.