(Note: This is part of a formal debate)
Euthanasia is suicide and sucide is indeed selfish. Suicide is focusing solely on your own circumstances and on your own pain to the degree that you don't want to put up with life anymore. Its the crowning achievement of selfishness.
Everyone lives with pain, so at some point you must force some people to live in pain. Pain is something that defines living and life, therefore every life will contain some pain.
Even if you accept your premise, there must be an amount of pain that is not justified and an amount of pain that is. Getting sunburn for you non-black people is painful, yet we would agree that sunburn is no justification for euthanasia. At some point there has to be a distinguishment between pain you have to live with and pain that justifies euthanasia even if we accept all of your previous arguments.
Secondly, there is a social duty to live if you in fact have social duties to fulfill. There have been many tales of soldiers who could have given up on life but instead lived through excruciating pain to fulfill some social obligation. The runner of Marathon comes to mind. So even then there are some cases that there is a social obligation.
Thirdly, wallowing in self pity to the point where you focus solely on your pain rather than anything else once again points out the selfish nature of suicide. Sucide focuses solely on the person who contemplates or does it, and hardly anyone else without the prism of "me, me, me".