Nothing in the first lines tells you who is speaking, and that's part of the fun. The speaker says "I play with you" and "I stay with you," and promises to do anything for their owner, so it reads like a devoted friend or a child. Only the last word shows it's a dog talking, and the title, "Mans Best Friend," quietly hints at it all along.
The speaker feels deep loyalty and a sense of belonging. Being treated "like one of your children" matters to them, and the blunt line "Because I am" shows they don't doubt it for a second. The middle stanza is the most honest part. When things get "rough and scary," the dog admits to acting "a little wack" and even attacking, yet trusts that love won't disappear. The mood is warm, simple and affectionate, with a touch of humor in the casual wording. The short, plain lines and the final reveal give it the feel of a heartfelt confession from a pet.