It's really not. Since toddlers don't really care where they go, just like an untrained child can have an accident in his pants, so can he have an accident into a potty. All he needs is the opportunity. Opportunity is the key. But just as important, keep it pressure free.
Children love to please their parents.
They love to do new things and feel a sense of accomplishment for their successes. If we demand things of them that they're not capable of or not ready for, it should come as no surprise that they don't cooperate. He should be ready to sit on a potty or toilet. The first step in potty training is to have a child sit on his potty regularly. If a child refuses, express mild disappointment, but don't insist.