How can you be sure that your assumptions of the condition of the runway won't allow for a succesful aborted takeoff before V1?
Can you make that decision in 2 seconds?
Fire warning - one, two - did you continue or reject?
Do you then brief it in your take off emergency briefing or is it something you decide if it happens, that you will continue before V1?
How do you know how bad the fire is to the engine and that the engine is not severely damaged by the outburst of a fire and then producing no thrust?
What is your new decision speed if you don't want to use the one calculated for you by the fmc, V1-1kts V1-2kts V1-3kts V1-10 kts V1-20 kts?
Where does your SOP allow you to disregard calculated FMC speeds and use one made up in your head?