A lot of people think that it is because of the love they feel for their tutors and to kill the homesickness of their humans, when they spend a lot of time outside.
But beyond love, dogs' willingness to cling to their favorite humans has a lot to do with instinct.
According to experts in canine behavior, one of the first things dogs learn to do while still babies is to follow their litter siblings. It may pay attention, when a puppy begins to crawl towards its mother, the others all begin to follow, all intending to reach the power supply, which in this case is the milk.
When they come to live with humans, the dogs begin to see their guardian as the alpha of the pack, the main and strongest member of the group, after all, it is he who provides shelter, security, affection and especially feeding for the dog. And then, by instinct, they will follow this human everywhere.
Besides all this, the dogs are animals that enjoy company and to be part of a community, be it canine or human. Being alone is not with them.