I am a sentientist because I believe that no living being is inherently more valuable than another based on immutable traits: race, gender, age, or species. The value of life stems from pleasure, so to enslave or kill a being which can feel pleasure, or might be able to in the future (note that this includes nonsentient embryos and fetuses, but not a being which is braindead or cannot feel) is to rob it of all potential future pleasure.
@Arachnocat14