Taking your pup for a walk should be an enjoyable experience, not a frustrating one in which you're attempting to pry open his mouth and retrieve the bottom of his leash. But dogs will be chewers, and some just can't help themselves. Determining the root cause of the behavior and kicking it to the curb will help you set your little guy straight.
The Ins and Outs
Young canines tend to chew just about everything, including leashes. If your puppy is between 3 and 6 months of age, he's probably chomping down because his baby teeth are falling out and those adult teeth are barging in. Puppies and adult canines alike tend to react crazily to new things, and especially things they don't like. A lot of times this is out of fear, and sometimes it's out of frustration. If your pup cowers from the leash or freezes up when he sees it, he's probably biting it to try and get it off. If frustration is the culprit, he'll sink his teeth into the leash as soon as something he doesn't like happens. For example, when you don't let him run full speed ahead, or you tighten up on the leash, he might become agitated, throw his head back and bite down on the leash in attempt to break free, or at least let you know that he's one unhappy pup.
Whether you're dealing with a puppy or a fully grown canine, if he's chewing on the leash because he's afraid of it, it's time to calm his fears and make him see the leash as a minor accessory. Your tools of choice include treats and a positive, uplifting voice. Bring out the leash, saying not a word, and drop it on the ground, and then have a seat next to it. The second your pup brings his investigative self to it, toss him a treat. Keep doing this until he's completely comfortable around the leash, and then keep upping the ante, using this experience as your guide. For example, attach the leash to him, and give him a treat and let him walk around with it in the house, but always keep an eye on him. Take baby steps, rewarding each time he reacts positively to the next step. If he freaks out, go back a step.
Putting a Stop to Frustration
If your pup is trying to show you he's mad when you keep him from doing fun things, such as chasing a chipmunk or sniffing another dog's butt, put your foot down. He doesn't get to do anything until he drops the leash. Don't yell at him, just stand there, refusing to move. The moment he drops the leash, let him sniff the telephone pole or whatever he's after -- as long as it's safe. Treats aren't typically necessary for this, because doing what he wanted to all along serves as his reward. Never give in and let him charge toward his destination until he drops the leash, or he'll think he can chew on the leash to get what he wants.
Spraying the bottom of the leash with a bitter apple spray sometimes prevents eager mouths from latching on. Another option is to teach your pup the "leave it" command so he drops the leash each time you say the command. Some dogs prefer to have something in their mouths much of the time, so they pick up the leash and just carry it with them. Bringing a tennis ball or one of your pup's favorite toys along can help with that. Chain leashes can help deter chewing, but avoid using them if your pup bites down hard on the leash or is a puller you have trouble controlling sometimes.