As is true with any dog breed, the St. Bernard needs socialization in order to be a well-rounded and accepting dog. He is naturally friendly, but he needs to learn the proper manners he needs to be around other people and animals. The basic reason for this is because of his size. Large dogs don't often know instinctively that their size can be an impediment to their "friendships" with strangers, and they need to learn to control themselves when they meet new people and animals.
The first thing to remember about your St. Bernard before you begin socialization training with him is that he is not an aggressive breed. In fact, though he makes a terrific watchdog, and will bark at strangers to alert the family of their presence, the St. Bernard simply wants to be friends with the stranger, no matter who he is. This is a great benefit of the dog's personality, especially if your home is open to a lot of company and visitors. The problem is that the St. Bernard's size and his intimidating bark can put strangers off until they get to know the dog. Therefore, your St. Bernard needs to learn how to control himself when he is in a social situation.
The St. Bernard will love being around new people and animals. He is an incredibly social dog, and because he does not have any aggressive tendencies, he will not have any territorial aggression. This means he will not fight with or growl at any new dogs, male or female, which are in his home or territory.
The best way to socialize your St. Bernard is taking him to puppy obedience classes and puppy kindergarten. There he will learn the basic puppy skills like sit, lie down, and stay. He will learn to obey you and he will learn to be around other dogs. His needs to know how to stay down and not jump on strangers or animals before he can be allowed to go to the dog park or any other area where he will meet new people and dogs; his size alone could hurt even a grown adult person.
The same is true for the St. Bernard's manners around children. He needs to be very well behaved before he can be allowed around children, but not because of his temperament. He just needs to learn to control his body so he doesn't accidentally hurt a small child.
The St. Bernard is very good with other animals, including other dogs, but the same rules apply to new animals as applies to new people. His size is his enemy when it comes to smaller pets, so make sure he is well-trained before he is allowed to play freely with other dogs.