Wanting to live
Sim discovered that an experimental enzyme might extend his life beyond the fifteen days to the length of a normal human lifespan. Phlox told him that there was no proof that the enzyme would work and the time it would take to determine if it would work would age Sim beyond the point where he could donate the necessary tissues to Tucker, with the result that Tucker would die if Sim was saved or waited too long. Sim thought about stealing a shuttlepod to escape, and went as far as setting up the computer to aid in this escape, but he realized that if he did this, then Tucker would die and the Enterprise's mission to stop the Xindi might fail, causing the annihilation of mankind – not to mention that Sim would have to live the rest of his incredibly short life alone in space, "peeing in bottles" because the shuttlepod was not equipped with restroom facilities. With this in mind, he decided to submit to the operation. T'Pol kissed him goodbye and he reported to sickbay.
