I took him to a dog training class and asked the trainer about his behavior. He assured me that his "defensive mode" was just him being protective and curious. My dog is toy driven, so he advised me to distract him with a toy or pet him to let him know that everything is okay. Thank you guys for the comments! I have invested in a harness, and I've noticed that he seems to walk with more ease when wearing it. He still sniffs around (but from what I've read from the comments, it must be pretty common).