I should probably clarify that my motivations are entirely selfish and have nothing to do with altruism or sense of martyrdom.
I want to be able to dictate the image people have of me - the thought that a loved one's enduring memories of me is of as a burden (even one they willfully asume), or as someone that isn't capable terrifies me.
With respect to
@Phlog 's second point, I actually grew up in the exact opposite circumstances (unwavering love and support from family), which is arguably why I place a much greater emphasis on obligation to loved ones, than their obligation to me.
I would drop and do anything to take care of someone close to me, but I don't place a particularly high utility value on living a "long" life myself. I want to live by my terms, my way - hopefully that entails 90+ years of vibrant and healthy living, but should I be faced with a situation of facing some sort of terminal illness or injury (sufficiently so where I rely on others to care for me), I am taking the cowards way out and 'leaving early'.
Who knows, maybe I am talking out my ass, or maybe my opinions will change when I have kids who depend on me to stick around, but as of right now, I genuinely would not want my partner to stay with me if I got sick.