In very, very, very rare cases, suicide can be justified. We're talking "I'm dying, there is no cure, I'll be in incredible amounts of pain for the rest of my life."
Other than that, suicide doesn't just effect the one who commits it. It effects everyone around them in one way or another. Think of it this way...
There's a big puddle. Let's say 8 feet across, perfectly circular for the metaphor's sake. The person who commits suicide is in the dead center of it (no pun intended) and the parts closer to the center represent the people closest to the suicidal one. The person commits suicide; you throw a huge rock right in the center. The entire puddle is thrown into chaos (pain, sadness, grief), some water splashes out (people leave what could have been a close knit group), and the puddle probably deforms and it no longer a perfect circle (everyone's life from that point on is either a little messed up or completely fucked up).
... Probably not the best metaphor, but the point still stands. Suicidal tendencies are usually signs of mental disorders that can be helped, and those that go through with it irreparably change everyone around them.