Even though brake jobs typically aren't (strictly speaking) all that difficult, a lot of amateur mechanics like to hire a pro for this kind of work. And it's understandable. Screwing up a brake job can have pretty serious consequences, for obvious reasons. Your car's brakes work because of friction. It's not entirely that simple, but that's the basic principle. When you press (or slam) the brakes, hydraulic pressure in the system makes your brake calipers and brake pads squeeze in, and this friction causes your wheels to slow down.
That's a lot of friction, and it depends on a lot of moving parts. Moving parts require lube, or else they'll seize. So when you're taking brake components apart and putting them back together, you'll need to use brake-system-specific grease to make sure everything's operating at top-notch capacity. And if you're working with the axles, there's a strong chance you'll be using oily substances in the brake vicinity. Either way, you'll want to be extremely careful with lube application -- it's crucial. It requires the precision of carving a really expensive steak, or giving someone a tattoo. If axle grease or caliper lube gets on the friction surface of the brake pads or rotors, your car's brakes won't work. Not at all.