I remembered reading this on yahoo once I read pkmn's topic.
http://blogs.chicagotribune.com/news_columnists_ezorn/2009/05/your-dog-does-not-love-you-and-other-cold-nosed-truths.html
Pretty interesting if you think about it. When I went on vacation, my neighbor took care of my dog (went twice a day to feed him and let him out to relieve himself). He seemed very excited when we came home. He jumped in the car and wouldn't get out, which is what I guessed was because he didn't want to be left alone again.
Of course, he may have been doing that not because he missed us, or just because he was bored in an empty house for three days.
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