Repairing a roof isn't recommended for a do-it-yourselfer for one reason -- it's easy to fall off of. Repairing a roof shingle or two isn't the toughest job in the world, but it's getting up and down and carrying your tools with you that pose the risk of injury or death. It's also very tiring work, and when you're tired, you're more prone to make a mistake. Just a quick slip is all it takes to send you over the edge of a second-story roof.
If you live in a one-story ranch and your slope is less than 20 degrees, you can probably get away with gutter work and minor shingle repair. Your roof may even be low enough to do it from the ladder. But these minor fixes still can be dangerous, and you should never attempt any of them when you're home alone. At the very least, you should have a spotter in place to hold the ladder and be there in case of an accident. Aside from the danger involved, roofing work also requires experience to get it right. If you bite off more than you can chew, you may end up with a leaky roof and expensive water damage.