Protecting the weak from those who would dominate or kill them is just the right thing to do.
If you’re a good character, you believe it is right to aid and protect those in need. You’re not required to sacrifice yourself to help others or to completely ignore your own needs, but you might be asked to place others’ needs above your own . . . in some cases, even if that means putting yourself in harm’s way. In many ways, that’s the essence of being a heroic adventurer: The people of the town can’t defend themselves from the marauding goblins, so you descend into the dungeon—at significant personal risk—to put an end to the goblin raids.
You can follow rules and respect authority, but you’re keenly aware that power tends to corrupt those who wield it, too often leading them to exploit their power for selfish or evil ends. When that happens, you feel no obligation to follow the law blindly. It’s better for authority to rest in the members of a community rather than the hands of any individual or social class. When law becomes exploitation, it crosses into evil territory, and good characters feel compelled to fight it.
Good and evil represent fundamentally different viewpoints, cosmically opposed and unable to coexist in peace. Good and lawful good characters, though, get along fine—even if a good character thinks a lawful good companion might be a little too focused on following the law, rather than simply doing the right thing.