Head gaskets fail for one of three reasons --

1. ENGINE OVERHEATING CAUSED BY

A sticking thermostat.

Failed or leaking water pump.

Blocked or leaking radiator.

Failed radiator cap.

Leaking radiator or heater hoses.

Leaking heater core.

Radiator fan or fan clutch failure.

2. ABUSE CAUSED BY

Detonation or pinging (failure to properly time or tune the vehicle)

Racing.

Racing fuels.

Abusive driving.

3. DESIGN OR ASSEMBLY PROBLEMS

Poor gasket design.

Improper gasket installation.

Improper engine assembly.

In addition to repairing your head gasket, your mechanic should diagnose the CAUSE of the failure so that it can be corrected. Failure to correct the underlying problem will lead to future gasket failure.