Your dog has his own personality and quirks, which is why you love him. His behavior is often fairly predictable -- you know if the doorbell rings, he's going to bark, for example. When you notice rapid changes in your pooch's behavior, it's time to find the source of the changes to make sure he's as happy and healthy as he can be.
Behavior changes can be the first sign of a serious illness, so keep your eyes on your pooch if you notice him acting strange. This might include forgetting his house-training and having accidents indoors for the first time in years or refusing to eat anything but canned food. He also might snap at you, especially when you touch him. Although these otherwise unexplained behavior changes might be something as simple as your pup's upset stomach, the changes might be triggered by something more serious, such as diabetes or cancer. Always get him checked by a vet, just in case.
As your dog matures, his behavior should mature as well. However, senior dogs sometimes start acting differently over time or suddenly. Some of these behavior changes can be disturbing to you, such as if your pooch starts barking for no apparent reason or sits and stares at a stationary object for minutes at a time. Your furry friend might become way too clingy, never leaving your side when you're home. Or, he might decide to sleep all day and roam the house all night. Some of these behaviors result from trouble hearing or seeing -- which you might understand better in a few decades -- or from the fact that his brain is starting to slow down as he ages. A vet can help you determine whether the new behaviors are age-related.
Introducing new dogs into your household is more difficult than just letting them sniff each other's private parts. Dogs experience jealousy just like you can, and they don't always like little whippersnappers taking away their attention. This can result in dominance aggression, where your pooch tries to take a chunk out of the new pet on a regular basis, or it can cause your pup to revert back to his younger days, when he chewed your shoes and ate entire toilet paper rolls. Mitigate this a bit by introducing new pets gradually. Let them meet in a neutral spot, such as a park. Keep them both on leashes, but let them sniff and play. Let them play in your yard together before bringing them both inside, and have separate feeding areas for them. Your routine should stay as normal as possible, letting your dog know that the new addition is the only change that's happening.
Some dogs, especially ones that are adopted from a shelter and perhaps weren't socialized adequately as puppies, don't handle change well. Your pooch is a creature of habit, so even little things such as moving his dog bed to the other side of the room can wreak havoc with his behavior. He's as likely to pee on his bed at that point as he is to sleep on it. Other changes that can affect his behavior include moving in someone new, such as a roommate or family member, or selling your home and buying a new one. Even the smell of a new couch can cause your pup to chew on it rather than adopt it as his own -- at least at first. With some time and patience on your part, you can help him come to terms with changes and get back to being his own lovable self again.