Leonbergers are extremely affectionate and social dogs who live to be next to you constantly. They will follow you everywhere you go, from the bathroom to every other room in your house, just to be next to you. Some lick more than others to show you their affection.
Leonbergers perform wiggly and bouncy (read: very energetic) dances of joy when you come home, and every time they see you after an absence, however short it may be. Some will add vocal intonations to this dance of joy that would make an opera singer envious. Without proper "greeting behavior" training, they will also jump on you to show you their affection. If you are not prepared to deal with this practically every day of your lives with them, then don't get a Leo.
Leonbergers shed an incredible amount of hair twice a year for around six weeks, and plentifully the rest of the year. You will find soft piles of Leo fluff in various nooks and crannies of your home, outside and inside your air vents, on all of your clothing, and very often, in your food. If dog hair bothers you, don't get a Leo.
Leonbergers have strong active tails that will knock things down, wipe things off your coffee tables, make the loudest racket as they bang on your walls, and give you the occasional welt.
Leonberger puppies go through an unbelievably destructive chewing phase. You will lose parts of your house-inside and out, your water hoses, sprinkler systems, all your plants, parts of chairs, and any other prized possession within the puppy's reach, UNLESS you are dedicated to keeping the puppy busy with appropriate chew toys, and in a safe puppy-proofed area AT ALL TIMES!! And even if you do absolutely everything right: if you are not prepared to put your Leo puppy first, and the destruction he/she may inevitably cause second, then don't get a Leo.
Leonberger males go through a protracted period of adolescence during which they may or may not get along with other intact males. Proper early socialization and consistent positive obedience training is absolutely essential for you to make it through this stage and end up with a mature stable Leonberger adult male. Leonberger males generally hit maturity at around 3-4 years of age. If you are not willing to take the time to train the giant-sized "handful" that a Leonberger male can be, don't get a male Leo!
Leonbergers have a "dry" mouth without loose flews, so they do not drool like some breeds such as Mastiffs, St. Bernards, and Newfies do. They do, however, dribble water after taking a drink, and they will dribble that water all over you, your clothes, and your furniture. Leonberger puppies also love to splash in their waterbowls and dump them out on a regular basis so they can lie down in the water. If these traits will bother you, don't get a Leo.
Leonbergers are very smart and intelligent dogs. While they thrive merely by being a daily part of your family, they will truly blossom if you keep their mental faculties sharp by participating in any working dog activity with them. Regular obedience, agility training, therapy visits, water rescue training, and drafting are all wonderful ways to keep active with your Leonbergers. If you don't have some sort of plan to keep your Leonberger's mind active, don't get a Leo.
Leonbergers, especially when young, need ample opportunites to let loose their energy. This can come in the form of play dates with other puppies, socialization time in puppy classes, regular walks with you, or swimming often in lakes or rivers. If you don't have a plan for exercising your Leonbergers, don't get a Leo.