What is love to you? What is sadness to you? What is life itself to you? What is purpose or meaning to you? What's the point of accumulating knowledge and experiences when it means nothing after death? Why do anything when it has absolutely no merit for you after you die? What value does life have for you, if mankind's existence was just some random occurrence? If you're going to say that we give value to our own lives, then what good is that value? Are you going to say that we should place value upon that value? What good is that? Isn't it ultimately pointless to keep placing on anything when that same value is meaningless in the end? Why do anything? Is just because we can? What is free will to you?
That is a lot of questions... many I feel may be unanswerable, either because there simply is no answer or I'm not smart enough to know the answer.
Love is a feeling you get when interacting with someone you care about, love is a combination of chemicals in the brain (the release of pheromones), love is showing empathy, love is being selfless, love is commitment.
For any society to grow and prosper, for relationships to flourish, love is necessary.
Sadness is a feeling you get, sadness is the release of chemicals in the brain, sadness is a byproduct of love in many aspects.
Sadness is a negative human feeling that tells us to strive not to be sad.
Life is the energy in our brain, that powers our body, that enables our consciousness.
The act of living is the meaning of life itself.
To live is better to not live.
Our purpose at the most basic level is to live, to survive, and have more good moments than bad.
A grander purpose is to learn and pass on that learning so that more people can benefit and then more people will have more good moments than bad.
The fact that things may hold no value after death is irrelevant to the present. We should do things for the merit they give us in this life. Its not pointless, it just may seem pointless when looking at it from an eternal perspective.
Also we don't live in a vacuum, the things we do may have merit to others even after we die. This is true selflessness, working to benefit those who remain even though it does us no good when we are dead.
As far as why do anything...well try not doing anything for any period of time. We are active beings with an active mind, simply do things for the sake of doing things if there is no better reason.
I will not get into free-will here because its highly controversial and debated among both religious and non-religious.
Please dont over scrutinize these answers, they are just my honest opinions off top of my head.