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The Shuttles are extremely complex pieces of machinery, with literally thousands of parts and extremely volatile chemicals (solid and liquid) that must function properly together in order for a successful launch to occur. As such, humans cannot monitor and control the many critical systems and react to problems fast enough during the final stages of launch, so the process is automated.

At T minus 9 minutes, a hold is placed on the countdown, and the Shuttle launch sequence is turned over to the Ground Launch Sequencer (GLS) software program at Kennedy Space Center. Upon resuming the countdown, the GLS system is responsible for monitoring critical launch systems for any failure (real or not), or parameters that are outside of established limits, interrupting the launch sequence if any parameter isn't met. At T minus 31 seconds, launch control is switched from the GLS system to the Shuttle's onboard main computers, which is then responsible for detecting any problems and stopping the count if necessary.

The NASA Launch Director has the authority to override an interrupt. This is done only upon analysis of the error detected after the launch interrupt, if the error is found to be either false (computers aren't infallible either), or the parameter which led to the launch abort is deemed to be small enough to be worth the risk (it has to be pretty small - safety of crew and vehicle are the first concerns). Depending on the severity of the problem, the Launch Director will either continue the countdown, or delay the launch in order to further investigate/fix the problem if it's severe enough (system failure, etc.)

It is not unusual for a Shuttle launch to be delayed due to a GLS abort; during my Kennedy Space Center visit to witness Discovery's launch (anyone who works on a Shuttle payload is entitled to view the launch on-center at KSC), which had no problems, I met a woman who had previously been to no less than 10 other launches. The one I met her at was the first she had actually seen go through, as all the others had been delayed either by computer aborts, weather, or the other many factors that are taken into account before a Shuttle can launch.

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Q: Why do computers determine if Space Shuttles will launch or not?
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