Each individual assigns value to their own life, I think. However, the default assumption is that everyone values life, and that it is something which is inherently scared and precious no matter what. In some situations, this is a good thing, because it means that an unconscious drowning victim can be saved, or passerby can be protected from the fiery hail of a nearby car crash.
For those of us who have ascertained that our own lives hold no value, this assumption becomes problematic. Serious self-harm is defacto illegal. Yes, you won't be criminalised for attempting suicide, but your decision certainly won't be sanctioned, and surrounding actors will do everything in their power to keep you alive at all costs, even if you don't consent to this.
We haven't reached a collective ethical and moral agreement that quality of life is more important than quantity. The basal state of living and breathing is considered to be valuable and holy, regardless of the circumstances which that living creature must endure. There is more to life than simply existing, and we know this as a species, hence why people scour for meaning, distractions, and ways to keep themselves occupied so that they don't ponder these extesential questions too much.
Many times I have been told x or y is for my wellbeing, and that I should not be bothered by certain things because they are deemed necessary for life to continue. How many times have you been told that life isn't fair, life is cruel, and sometimes we have to do things we don't want to do? To that, I pose the question, why should I have to do these things to continue a life I don't want? A life that I have deemed invaluable long ago.
I don't see life as inherently valuable, rather, it's the components and pieces that make a life truly worth living that are valuable. Once you don't have any of those things, why are you not allowed to determine that your own life lacks purpose, if you've set a metric for what value means to you? Indeed, all life comes to an end eventually, so it doesn't make much sense to impose one's personal philosophies surrounding bodily autonomy onto someone else, who doesn't agree with the default religious-based santicty of life belief system.
We're not at a point of consensus yet where these beliefs aren't seen as controversial or shocking, because it is hard for people who value their lives on such a deep level to conceptualise how it feels to get to a state where life is meaningless. Many people believe that devaluing your own life will lead to others following suit, hence the controversy over MAID being legalised in countries like Canada.
If the majority values life and thinks it is moralistic to do so, it is always going to seem unethical to them for someone to reject this or frame it as a pathology/illness. Many laws are founded upon the concept of duty of care/duty to uphold right to life, so even if people disagree with this on an ethical level, you can't do much when your jurisdiction has decided it is illegal for someone to condone your thoughts about suicide. Even if the other party secretly agrees with you that life is not always valuable, they are terrified of the legal and social reprocussions of expressing this.