Energy.
Chihuahuas have a ton of energy packed into their tiny bodies. You can think of it, like drinking too much caffeine. If your body is hoarding a lot of excess energy, it burns it off by shaking.
Attention.
Neva loves attention. She will jump on my lap and look up at me (just like she is looking in the picture) and she will shake. I usually respond by baby-talking to her. She loves for me to acknowledge her and I always respond when she shakes, so she probably knows she can do it by shaking. I’ve read somewhere in the past, that they have evolved to shake, because people feel sorry for them and pick them up. I don’t know if it’s true or not, but it makes sense.
My Chihuahuas Blog 
Coco is a drama queen, I swear at times she does it so she gets picked up,. Or her own way. My son will come over with his Pit, and if the pit eats out of a certain bowl she looks at me and starts shaking away, i give the pit a different bowl and Coco stops shaking.
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My puppy is 12 weeks old and bits me all the time. How do I stop it?
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Ours did as well and will continue until it looses its baby teeth . Now she just licks us to death. we used a firm voice command of No and just try to discourage the pup from doing so.A water bottle seems to work for the nipping at the feet . Good luck
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