The main reason that a brake goes soft is due to a leakage of brake fluid. It is important that the brake fluid is level and of a good consistency to ensure that your brakes do not fail and incur expensive repair charges. Check the fluid levels to ensure they are correct. Also, try changing the brake fluid to rid it of moisture and air. The brake master cylinder could cause your brake to go soft. Regular brake checks can prevent your brakes going soft. Regularly changing the brake fluid will prevent brakes going soft. A mechanic will be able to identify the reason for your brakes going soft if you are still unsure.
For more info, see Understanding the Brake System with a Brake Repair Diagram