After Rover and Connie are finally reunited with each other, it's just so sweet to see how happy they are.
Also, after Connie discovers what her boyfriend, Rocky, did to Rover (namely trying to kill him by dumping him over the Hoover Dam) and Rocky tries retaliating against her, Rover and his Vegas-friends protect Connie by chasing Rocky out of the casino, and Rocky ultimately gets what's coming to him from a group of fellow gangsters.
The ending of the movie is this (though also combined with Tear Jerker to a certain extent):
When Rover starts missing his life on the farm (including Daisy and the other friends he made back on the farm) and becomes depressed, Connie sees this and, ultimately just wanting her dog to be happy, takes him back to the farm, where's he reunited with Daisy (who reveals that she gave to birth to a litter of six puppies, of which Rover is the father) and his other friends. Though sad to ultimately go their separate ways, Connie and Rover share a heartfelt goodbye before she leaves to go back to Las Vegas.
The movie ends with Rover teaching his kids (five of whom look just like Rover while the sixth looks identical to Daisy) how to play cards and playfully chasing Daisy around the farm.