Theory of moral standing of virtual life forms

I have killed three dogs in Minecraft. The way to get a dog is to find a wolf, and then feed bones to the wolf until red Valentine’s hearts blossom forth from the wolf, and then it is your dog. It will do its best to follow you wherever you go, and (like a real dog) it will invariably get in your way when you are trying to build something. Apart from that, they are just fun to have around, and they will even help you fight monsters. If they become too much of a nuisance, you can click on them and they will sit and wait patiently forever until you click on them again.
I drowned my first two dogs. The first time, I was building a bridge over a lake, but a bridge that left no space between it and the water. The dog did its best to follow me around, but it soon found itself trapped beneath the water’s surface by my bridge. Not being smart enough to swim out from under the bridge, it let out a single plaintiff yelp before dying and sinking. Exactly the same thing happened to my second dog, as it was this second episode that made this particular feature of dogs clear to me. I know now to make dogs sit if I’m building bridges. I’m not sure what happened to the third dog, but I think it fell into some lava. There was, again, the single yelp, followed by a sizzle. No more dog.
I felt bad each time, while of course fully realizing that only virtual entities were being killed. 

From an excerpt from Charlie Huenemann's How You Play the Game: A Philosopher Plays Minecraft. It is true, we humans are wired to form attachments, even to inanimate objects or virtual life forms which have no feelings. We visit the places of our youth and experience almost nauseating waves of nostalgia. At an objective level, these seem irrational, even as we recognize the tendency in ourselves. What is the value of these feelings? Is there an evolutionary purpose? Are they a form of rehearsal for our attachments to living things, or maybe just a reflex we can't turn off selectively?

The point is that we form attachments to things that may have no feelings or rights whatsoever, but by forming attachments to them, they gain some moral standing. If you really care about something, then I have at least some initial reason to be mindful of your concern. (Yes, lots of complications can come in here – “What if I really care for the fire that is now engulfing your home?” – but the basic point stands: there is some initial reason, though not necessarily a final or decisive one.) I had some attachment to my Minecraft dogs, which is why I felt sorry when they died. Had you come along in a multiplayer setting and chopped them to death for the sheer malicious pleasure of doing so, I could rightly claim that you did something wrong.
Moreover, we can also speak of attachments – even to virtual objects – that we should form, just as part of being good people. Imagine if I were to gain a Minecraft dog that accompanied me on many adventures. I even offer it rotten zombie flesh to eat on several occasions. But then one day I tire of it and chop it into nonexistence. I think most of would be surprised: “Why did you do that? You had it a long time, and even took care of it. Didn’t you feel attached to it?” Suppose I say, “No, no attachment at all”. “Well, you should have”, we would mumble. It just doesn’t seem right not to have felt some attachment, even if it was overcome by some other concern. “Yes, I was attached to it, but it was getting in the way too much”, would have been at least more acceptable as a reply. (“Still, you didn’t have to kill it. You could have just clicked on it to sit forever….”)