If I don't ctb, based on my family history, my passing will likely either be a heart attack, dementia, or cancer. None of which are especially pleasant passings.
The painlessness or lack thereof isn't too much of a factor for me one way or the other. Were I set on catching that bus, I could deal with the pain if the method were surefire enough; it'd be over soon, right?
What keeps me off that bus is that I have people around me who would be very much upset by my passing. Most of all, my child. Even when she's a self-sufficient adult, though, dying by my own hand would be traumatic for her if we have any sort of relationship in adulthood (and I certainly hope we do!). If I die of old age, at least she can prepare herself when the time comes. A 40 year old dying is out of left field. A 90 year old dying is what it is.