To answer 'why' questions effectively, focus on the character's motivations and beliefs. Lencho's simple and absolute faith in God is the key to understanding his suspicion of men.
Lencho called the post office employees 'a bunch of crooks'.
He did so because he had an unwavering faith in God and believed that God could not make a mistake. When he received only 70 pesos instead of the 100 he had asked for, he immediately suspected that the employees at the post office had stolen the remaining 30 pesos.