Dogs and puppies are a lot like kids.  You have to watch them and keep them from harm because they don't think their actions through just as a young child does not think their action through.  A dog runs into the road because it sees something interesting on the other side just as a youngster might run into the road because it is focused on something on the other side.

I'm sure this well catch some flack but some people who are pet lovers form a similiar bond with their pets that parents form with their children.  You have to look out for them constantly, you hurt when they hurt and you want the best for them.  In short you love them, dearly.  And before someone jumps in to flame me on this - I am a parent and I am a pet owner (and animal lover) so I have experience on both sides of the fence.  In my opinion you can't just replace a pet no more than you can just replace a child.  You can get another pet that you will love and cherish with all of your heart but it will never replace the one you had before it.