Well, I would divide this into 3 levels as it is in games. Okay, starting from EASY level:
EASY:
"Sorry, I think our opinions differ so much that probably we won't understand each other if we continue discussion"
"I think you are right, but of course there are pitfalls and we cannot neglect them"
"This world made us different. This means that we can have different views and that is good. But how about... bla bla bla"
INTERMEDIATE:
"Honestly I don't think you are right in this situation, the light cannot exist without shadows and we were all born from chaos"
"You can always say that seeing copious harvest from your window, but the life may be a tragedy to those who see dark clouds instead of a sunshine"
"When you see the planes crashing, terrorists, famine, rapes worldwide, can you still say that life is amazing?"
HARD:
"If you think so, you are just a child of consumerism. Ignorant sheep which do not see what is on the other side of the field"
"If you believe life is something sacred, then the army is waiting for you. You will forget what is getting up at 12 o'clock, wear combat boots and hear the rumbling voice of a petty officer"
"Wait a sec, I will give you an address. This is a mental hospital, tell'em life is beautiful."
Okay, one more. Let it be extreme.
EXTREME:
"Well, you would better shut up before I started pushing in your mouth the reasons why you are wrong"
"I will now call a taxi, you punk go home and wash your mouth with the soap"
"Bud, if life was so cool to you, you would be now drinking expensive wine, eating lobsters and chilling on Bali instead of talking to me right now. Go get a job that you hate and then I will talk to you"
"See what, the universe exists almost 14 billion years. And this is the first time I hear such a bullshit from you. Is everything okay?"
"Do you know what is the difference between cow shit and bull shit?" -No - So if you don't fucking know anything about shit, why do you talk about life?