When I started, sometimes I had to go walk around the lab building afterwards to take a breath and gather myself. Now it still upsets me whenever I have to kill mice, but I'm used to it. I think that probably some scientists do get over it completely; the mice become tools. Other scientists hire technicians to handle and kill the mice. The researcher works with the cells of the mouse once it's killed, but never encounters the living mouse.
But that's not how we do things where I work. In fact, the people I get along with best in my lab sometimes even talk to their mice. While we're herding a mouse towards one side of a cage or another we might say, "Come on, sweetie." Maybe if we're injecting a mouse with something and it squirms, we say, "OK, OK. It's going to be just a second." When we put it back in its cage, we might say, "There you go." And while the mouse is sniffing and inspecting its cage mates, we say, "There's your buddies." I was in the mouse room with a colleague of mine and I noticed her doing this. And I said, "You talk to your mice, too." She said, "Doesn't everybody?"
It really captures the tension between mouse-as-tool and mouse-as-animal. And the -- well, I don't want to call it a happy medium, but the medium that most good scientists find. I dare say that working exclusively with cells, and never encountering the organisms they come from, has got to be dissociative. Seems like it'd be a good idea to keep that perspective in the back of your head, that these little spots in a dish came from a living animal, and serve multifarious purposes in that animal, and do things besides sit in culture and express your marker protein or what have you. They grow, they thrive, and above all, they are more complicated than they seem, even when you take that last into account.
A scientist who gets too debilitatingly upset over the death of a mouse will never get anything done, and a scientist who doesn't care about the mice as living creatures will have less perspective and, in all likelihood, get worse results for not being personally involved with their care.
I don't recall ever talking to the mice, but I didn't handle them all that much. Pretty much all I did was scruff and snip, and then they couldn't listen anymore. If I have to do more involved mouse procedures in the future, I probably will get in the habit of talking to the mice, especially given that I already even talk to inanimate objects when I'm working on them.