Happiness

Why is it that when I meet someone whom I wish well, I would care for his or her happiness?
Why is it that the first thing I messaged back to the person who had just received his results, is 'Are you happy with it?'
What is so important about happiness?
Is happiness necessarily something everyone strives for?
Or something that everyone ought to strive for?
And ultimately, in what, or in whom, shall I have the final happiness, the true joy, the greatest blessedness?

'The greatest happiness is for the mind to attain God.' - Saint Thomas Aquinas