B737 FLIGHT CREW REJECTS SECOND TAKEOFF; CARGO DOOR WARNING IS MEL'ED; BUT CREW FAILS TO READ MEL BOOK AND UNDERSTAND THAT WARNING IS TO BE IGNORED.

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ON TAKEOFF ROLL; AFT CARGO DOOR WARNING LIGHT ILLUMINATED. ABORTED TAKEOFF AT LOW SPEED AND CLEARED RUNWAY. WE WERE ASSIGNED A SPOT; NOT A GATE; AND MAINTENANCE WAS CALLED TO CHECK DOOR. SHUT DOWN #2 ENGINE; WHILE MAINTENANCE RECYCLED AND LOCKED DOOR. DOOR WARNING LIGHT EXTINGUISHED. MAINTENANCE CALLED ON RADIO AND GAVE MY FIRST OFFICER A MAINTENANCE MEL NUMBER FOR CREW TO WRITE UP DOOR AND PLACARD. I WAS GETTING CONCERNED ABOUT FUEL AND STARTED TO RUSH MYSELF. FUEL WAS NOW BELOW BLOCK FUEL BUT WELL ABOVE MINIMUM REQUIRED FOR TAKEOFF. FIRST OFFICER SUGGESTED WE WRITE UP MEL ENROUTE AND I AGREED. WE DID NOT LOOK AT MEL MANUAL AS WE ASSUMED PROBLEM WAS SOLVED -- DOOR LIGHT WAS EXTINGUISHED AND DOOR WAS LOCKED BY MAINTENANCE. RESTARTED RIGHT ENGINE AND TAXIED FOR TAKEOFF. ON SECOND TAKEOFF ATTEMPT; AFT CARGO DOOR LIGHT ILLUMINATED AGAIN AND ONCE AGAIN I DECIDED TO ACCOMPLISH A LOW SPEED ABORT (LESS THAN 60 KTS). RETURNED TO GATE AND GOT LOTS MORE FUEL AND APPLIED MEL FOR AFT CARGO DOOR. THIRD TAKEOFF ATTEMPT NORMAL WITH A NORMAL FLIGHT. ON THE SECOND TAKEOFF ATTEMPT; I DID NOT HAVE TO ABORT. THE MEL ALLOWED US TO CONTINUE THE TAKEOFF. HAD WE PROPERLY CHECKED THE MEL; WHICH WOULD HAVE TAKEN A FEW MINUTES; WE WOULD HAVE DISCERNED THE LIGHT MAY COME ON AGAIN; YET THE TAKEOFF MAY BE CONTINUED. I ALLOWED MYSELF TO BE RUSHED BY MY CONCERN FOR THE FUEL SITUATION AND ASSUMING OUR PROBLEM WAS RESOLVED BECAUSE THE AFT CARGO LIGHT WAS EXTINGUISHED. I SHOULD HAVE STOPPED THE OPERATION AND CHECKED THE MEL ONCE MAINTENANCE GAVE US THE MEL NUMBER. THE LESSON IS ALL MEL'S MUST BE CHECKED THOROUGHLY AND IMMEDIATELY AND ONE CANNOT ALLOW THEMSELVES TO RUSH AND CUT CORNERS EVEN IF THERE IS A FUEL CONCERN. IF I HAD A FUEL CONCERN; I SIMPLY SHOULD HAVE STOPPED THE OPERATION AND ASKED FOR MORE FUEL.