I felt exactly the same as you so can understand where you are coming from on this.

We have a five year old Bichon Frise and we wanted another dog but I was scared of spoiling things for him. He is much like your Westie, the perfect dog and is easy going and fits in with everything we want to do and I didn't want to upset him in any way by adding another dog but he does love other dogs when we are out. We took the plunge three weeks ago and got a little Bichon bitch and he loves her. It's lovely to watch them playing together and sleeping together. He used to move away at first when she slept next to him but now he doesn't. She is doing him so much good because I think he was getting old before his time - she has livened him up no end.
She was a pain for pulling on his ears and tail at first but she doesn't do it as much now and she loves him chasing her round the garden and he seems happy to oblige. He is neutered and she will be spayed when she is old enough.
I was looking at getting another breed of dog but figured that Benji seems to get on with other Bichons best when we see one on our walks which isn't very often he seems to want to play with them more than other dogs so another Bichon seemed the obvious choice.
Here she is with him in the garden pulling his ear.
On a walk together.
If he wants time away from the puppy if she does get a bit much he goes upstairs and sits at the top of the stairs where she isn't allowed.
