In an emotional thread over on Twitter, Joel goes into great detail on how the character was intended as a companion first and foremost, and a combat ally second.
Dogmeat In Fallout 4
For anyone who has ever wanted to understand how the role of Dogmeat was accomplished in Fallout 4, Joel's tweets really hammer home how the process worked. It's not your average casting story, but one that is filled with emotion.
Most surprising is that River was not a trained acting dog. Instead she just happened to be in the right place, at the right time, with the right incredible personality.
If you have ever owned a dog, then the companionship between your character and Dogmeat in Fallout 4 is an instantly recognisable bond. For anyone who hasn't owned a dog, Fallout 4 does an amazing job of representing that bond and relationship.
A Companion's Love
Dogmeat is never an aggressive character. His intentions are only ever to keep you safe, lending more of a purpose to the character than many other NPCs in the game.
While Dogmeat's actions can never be explained by the character, it is an easy assumption to make that their devotion to you character is what motivates them.
Even if you cannot connect with Dogmeat on an emotional level straight away, Fallout 4 helps you to understand that your character and Dogmeat are in the exact same situation. You are both out of your depth, living in a world you're not supposed to, and trying to survive the harsh wasteland.
This bond is what connects you to Dogmeat as the game goes on, and it's in River that Dogmeat's personality was born.
River's fur markings made for and interesting dynamic where she and Dogmeat could almost convey emotion and personality in their faces. This was key to being captured in-game and helped make Dogmeat the perfect companion.