My chihuahua, Walter is insane. He will literally kiss me one minute and full on bite me the next. He is a puppy mill rescue and has mental issues from it. I know it's bad, but I don't do much about it. After the mill, we had several teeth removed and even if he gets you it's not that bad. Anyone that handles him, knows his ways and at 7 lbs, he's not too much of a threat.

So to answer your question about why they bite, I clearly have no clue!

Personally, I wouldn't let Walter get that close to a stanger if I thought he'd nip.