Chief end of man is to Glorify God and enjoy Him forever' (shorter catechism).
For a Christian the answer is 'For God's Glory'. Let me explain. God
is absolutely pure and perfect in every way, otherwise He would not be God. When someone on earth does well and is good at what they do or have done..then we rightly recognise this by affirming them a degree of thanks and praise for their charactor/efforts.
Now..God is perfect and does all things perfectly so it is His right to be praised for this...in fact if God refused this praise He would deny Himself,for perfection demands admiration and praise. God is not
being selfish by wanting everyone to praise Him and 'glorify' Him, but He is simply being true to Himself. He cannot deny Himself.
God is also all Wise and wisdom is the right use of knowledge. God's knowledge tells Him He is perfect so His wisdom operates
on that premise.
So when God works all things in this world by operating His wisdom expressed in His providence,He is working to one great end which is His Glorification - that all men everywhere should glorify Him, which is His chief desire and
our chief end in life.
The creation displays and magnifies His glory. Christians show His glory in redemption and the unconverted, who reject His love in Christ will glorify God's perfect justice and
Satan and the rebellious Angels will also glorify God when He excerises His perfect justice and judgement in their condemnation.
So in all God will be glorified and His perfection will be acknowledged.
That is reason why we are here...it is for the GLORY OF GOD.