Social Contract?

Question:: 
Your justification for jesus' existence was thurough. However, the existence of a social contract (athiest moral stem) that is just innate in humans has even less proof of existing than jesus. How can you justify morality based on a guide that cant be proved? I understand how a social contract is constucted, but what purpose does it serve in the end? And if there is no purpose to existence, why are we alive?
Atheist Answer: 

Your question is asked in good ole theistic backwards style. The questions you ask have very obvious answers, the type that almost don't warrant answering. But in the spirit of AskTheAtheist, I'll play along. Truly the only reason I jumped in was so I could get another stab at linking to Jake's morality video. Jake is the creator of this site, if you're just joining the party.

Here are the answers to your questions (I'll answer them in a very basic style, k ?)...

How can you justify morality based on a guide that cant be proved?

How's this for a guide that can be proved... one person shoots another person, everyone who loved the victim is crying. Do you think the people crying are hurt and upset, maybe even devastated? Do you feel good when you are hurt and devastated? If you are like me, you don't feel good when you are hurt, therefore you try not to devastate and hurt other people.

I understand how a social contract is constucted, but what purpose does it serve in the end?

It keeps us alive as a society, otherwise we'd most likely all be dead.

And if there is no purpose to existence, why are we alive?

What book told you there was no purpose? The Bible? I've never known anyone in my life without a purpose, that's just some retarded bullshit nonsense that Christian terrorists pull on you to scare you into believing their bat-shit stupidity. Avoid them, they're con artists.

- Brian Sapient
Rational Response Squad