Causa vs. motivo vs. razón
In Spanish, the words razón, motivo, and causa all translate to "reason" or "cause", but they are applied in distinctly different contexts. The correct choice depends on whether you are describing an objective physical trigger, a personal internal motivation, or a logical and intellectual justification.
Causa
The word causa refers to the objective, physical, natural, or scientific origin of an event, representing the direct trigger that produces a specific effect.
La causa del incendio en el bosque fue un cortocircuito.
(The cause of the forest fire was a short circuit.)
Los médicos todavía están intentando determinar la causa de su enfermedad.
(Doctors are still trying to determine the cause of his illness.)
Las fuertes lluvias fueron la causa de las inundaciones en la ciudad.
(The heavy rains were the cause of the flooding in the city.)
El abogado demostró que no había relación de causa y efecto.
(The lawyer proved that there was no cause-and-effect relationship.)
Se desconoce la causa exacta del accidente aéreo.
(The exact cause of the plane crash is unknown.)
Motivo
The word motivo is related to personal motivation, representing the emotional, psychological, or conscious drive that leads an individual to make a decision or act in a certain way.
El principal motivo de su renuncia fue el bajo salario.
(The main reason for his resignation was the low salary.)
Me gustaría saber cuál es el motivo de tu largo viaje a Japón.
(I would like to know what is the reason for your long trip to Japan.)
La policía interrogó al sospechoso para descubrir el motivo del crimen.
(The police interrogated the suspect to discover the motive for the crime.)
Ella no tiene ningún motivo para mentir sobre lo que pasó esa noche.
(She has no motive to lie about what happened that night.)
Compró flores sin un motivo aparente.
(He bought flowers for no apparent reason.)
Razón
The word razón represents a logical, intellectual, or rational justification for an action, belief, or occurrence, often used when explaining the thought process behind a circumstance.
Dame una buena razón para no cancelar nuestro contrato.
(Give me a good reason not to cancel our contract.)
La razón por la que llegué tarde es que había muchísimo tráfico en la autopista.
(The reason I arrived late is that there was a ton of traffic on the highway.)
Pudo explicar la situación, pero sus razones no fueron muy convincentes.
(He was able to explain the situation, but his reasons were not very convincing.)
Por razones de seguridad, no se permite llevar líquidos en el avión.
(For security reasons, carrying liquids on the plane is not permitted.)
Esa es la razón principal por la que decidí estudiar otro idioma.
(That is the main reason why I decided to study another language.)
Summary
To summarize, use causa when talking about the objective, scientific, or factual origin of an event. Choose motivo to describe the personal, emotional, or psychological drive that pushes someone to take action. Finally, use razón when you are providing a logical explanation or an intellectual argument to justify why a specific situation happened.