Why is suicide considered to be wrong?

As someone who has lost a few friends to suicide I think its wrong because its a cowards way out. They all write letters that they say absolve their family and friends from what they have done but that doesn't stop the guilt we felt because of their actions. The last friend I lost, I'm still pissed at because he killed himself one week before his daughter's second birthday. She walked around asking "Where's daddy hiding?" thinking he'd pop out somewhere. She has to go through her life wondering why her dad isn't there. If he had died accidentally that's what we could tell her, but how do you tell her "Daddy didn't want to be here... with you, with anyone!"???