Is it about controlling people? Yes, but only in the same way all laws and regulations and social rules are.
I agree with the rest of your comment, but as for this paragraph, I actually think it's different.
For example it's a social rule that I shouldn't litter because then someone has to clear up after me. It's not a huge deal for me to just throw my empty can in the bin so I can follow this with no pain to myself.
Regarding suicide though, presumably no one is suicidal without a reason, whether that reason is mental, physical, or external - there is still some type of reason.
In telling someone not to end their pain because it will upset others, you are not only asking them to think about others, you are asking them to live daily through pain just so others don't feel pain instead. This seems to me a highly selfish request, and the fact we live in a society where we call suicidal people selfish is backwards to me.
Imagine telling someone slowly dying of a terminal illness "no you can't kill yourself because it will make me sad."
Nevermind the fact that person is in constantly physical and mental pain and can barely move.
To be clear I'm not advocating suicide as a good thing. Only questioning the social views around it.