Euthanasia violates and devalues the sanctity of human life
The sanctity of human life is the idea that human life has an inherent, non-relative value which should be protected and not violated for any reason, with exception of self-defence or defence of others. It's a major component of many major religious traditions and the basis for human rights.
The human life is sacred, irrelevant of its worth for the society, the market or its general usefulness or self-reliance. We cannot differentiate between "better" or "worse" lives based on its quality. Every human being should be treated as an individual, whose well-being and right to live is of the utmost importance, no matter what affliction ails them. Legal euthanasia cannot be accepted in the light of this as its most basic premise - that there are some lives not worth living anymore because of a suffering or an illness - undermines the sanctity of human life. This can have profound and undesirable consequences for the way we think about ourselves as human beings.
The sanctity of life is not an absolute law. For instance, many societies permit capital punishment or even passive euthanasia. This view also imposes certain moral considerations on individuals, who might not subscribe to them.
[P1] Human life is sacred. [P2] Euthanasia undermines this by asserting that life can become not worth living. [P3] Legal euthanasia cannot be reconciled with the sanctity of human life.
Rejecting the premises
[Rejecting P1] The sanctity of human life is not a universal view or without caveats.