From a religious point, suicide is considered evil, a sin. It is an act against one's community. It takes control of something that only God should have control. At first it doesn't seem hard to tell the difference between the two but it is. There are never ending arguments about the difference in these two phrases.
People depend on the medical community to do everything possible to save lives.
If instances came up where they didn't do everything possible and let someone die, the trust between patient and physician would be ruined. There are several guidelines as to what is what, to help physicians make hard decisions but the religious and moral reasons are different for different people. One possible scenario is in the case of a comatose adult. To the best of medical knowledge, the adult will never again regain consciousness. If medical treatments are withheld, the patient will die.