Of Human Bondage

Here is an extract from the book - Of Human Bondage by W. Somerset Maugham. One of his early works and largely auto-biographical. 
Doing what you want to do and existing for its sake are two pillars that keep falling on each other. It supports the need of your presence but can crush if you rest too easily on one and look away from the other. This paragraph here sums up well the need of these two pillars.