Because I don't want to die, but I suffer from persistent chronic pain from some form of inflammatory disease. The remaining things that it could be are more and more serious as others are ruled out. Depending on what the final diagnosis is, it's prognosis, my continuing pain situation and how/if it can be better treated, will determine when if and when I decide to take my own life.
For me, the only method that I currently entertain is barbiturate overdose, whether or is N or one of the others currently in use for attaining self deliverance. I have experienced enough tachycardia from anxiety due to the pain I experience on a daily basis, I don't want my remaining moments of consciousness to be overly uncomfortable. I'm in no rush and many options are available, from sourcing my own N, getting it through MAID if/when the laws are amended in Canada, travelling for MAID, or even synthesizing it myself. A group of laypeople from EXIT in Australia synthesized their own N and wrote instructions on how to do so. It isn't easy, but 5 people made enough for one dose each over a weekend.
Many could ask why not an exit bag with inert gas or one of the may CO options. They just aren't how I want my last moments to be. I like the idea of the brain slowing down to the point it no longer functions and then everything shutting down.