What is life for? Why are we here? What is the ultimate experience of being in human form? In my experience and perception, the core, the place from which everything else springs forth in having joy in this human existence is having the ability to accept what is. It doesn't mean giving up.
The answer to, the key to what living is for is to learn how to master your inner world first. It is mastering the ability to be disciplined in your responses.
The one dominant behavior that starts you on a path of true happiness is the great ability to accept life as it comes and not wasting time trying to control it. Taking charge of your negative emotional reactions is the gateway to a happy life by far.
You are the creator of your life. What you choose is your responsibility and accepting the consequences of your choices with grace and learning from your choices no matter how difficult you perceive the lesson to be is tantamount to being divinely inspired because you are awakening to and with greater awareness.
When you have arrived at the ability to convert challenge to opportunity, to take the time to think about inner peace instead of thrashing about like an infant, you have reached, you are practicing one of the highest reasons for being in human form: that YOU define your reality.
The answer to "what is life for?," is that you are a creator and you must own your creation. Getting that concept and being it brings peace of mind.