what is man's purpose?

Finally finished with all the work for Christmas, I thought it would be better since there's lesser work to be done. Well, things are far from being over.
Suddenly felt so mentally and physically strained.
Perhaps, I really din't rely on God enough, well, in the first place, I didn't even rely on Him. I have forgotten even how to pray.
Sometimes, I really would think, what is our purpose on earth? I'm sure life is more than material, money, love, or even perhaps just evangelism. Why do God put us here? Why did He created us in the first place?
In the book, purpose-driven life, it is mentioned that we were created for God's pleasure. I try to understand it by equating it to like why parents want to have children. It is pleasurable to have children, I myself would want to have children. When they smile, I would be smiling too. I want to give them the best. But why do I want them here in the first place? I don't know. So that I won't be lonely? So that my own parents won't nag at me for not having children?
Why need our existence?
If God really created us for His pleasure, why keep such a great distance from us? Why is He so far away?
Why punish us for things that Adam and Eve had done long ago? We might be their decendents, but we are different. Why so unfair?
Some people say God is unfair. How do I show them that this God is God and He is the One, when I myself is at times doubtful?