Firstly, it's not so much declaring a lack of purpose as recognizing a lack of predefined purpose.Exactly. But when you declare your entire journey to be without purpose, why would you seek a destination?
But to answer your question... If you want something but don't have it, it makes sense to seek it.
To really spell it out:
- I feel a desire to have a purposeful existence.
- I don't believe that the purpose of my life is predefined by an omnipotent intelligence.
- I wanted a purpose, was not given a purpose, so I chose a purpose.