From a purely biological point of view it might be an attitude that has arisen from base mechanisms such as the need to survive, and because we evolved as herd animals these mechanisms are pushed onto a collective level - "I have to struggle through life, so you must do too." Though it is just a hypothesis that was thought up while posting this, so it could be wrong.
From a non-biological point of view, and aiming more toward a philosophical/social perspective: if you leave this world before you grow old then you are thought of as going before your time. Dying "before your time" is a standard created by others to decide whether or not you still have potential to benefit them. Just like a tool that is wielded because it serves some function; only after that device has been worn out is it thrown away. But the tool does not have a say in any of this, because it is only an object to its user; just like people that think about leaving this world before they are permitted - a one-sided situation.
When a person commits suicide "before their time" they are effectively removing themselves as a stepping stone for the ambitions of others around them. If anyone and everyone were allowed to end their own lives right now, without judgement or stigma, then the social heirarchy would fall apart. There would be no low-wage workforce to provide everyone else with cheap goods, at the expense of their own well-being. There would be no one to bully and ridicule. There would no longer be any lower-status individuals to look down upon; being at the top of a social order only carries power when there is someone else below to use it on, so those that were once powerful would then be powerless.
Others can choose to discard you once you have outlived your usefulness to them, but at the same time you are not allowed to discard yourself out of your own choice. It is hypocritical.
"You exist because we allow it. And you will end because we demand it." - Sovereign (Mass Effect)