I think you need to get underneath your assertions a little. While all of your ideas about 'life, the universe, and everything' are philosophically acceptable ideas, they bare no resemblance with your experience of life.
You don't experience your life as being just a cluster of atoms interacting; nor do you experience your life as having no purpose - you might not experience some profound meaningful purpose, but you experience purpose in your everyday actions, e.g. catching the bus on time; getting something to eat; getting an A on a test, etc.
Even the notion of searching for a rationalization for our motivation implies we freely choose to enter into motivated states, and that we fully understand why he have chosen to do so. If we are just chemical sacks, then why not just say that the particular chemical sack called a person by the other person-shaped chemical sacks exhibits behavior that the other chemical sacks define as 'motivation'.
Even the view that the universe is devoid of meaning, random atoms interacting, infinitely large and uninterested is still a view. Its not wrong - but its not right either - it just a view.
So your question can only answered from within the narrow framework of a particular set of beliefs. Since no person experiences life as you describe it, then all were dealing with is a set of philosophical ideas - and while those ideas may make up the particular stance that a person chooses to take - if that person tries to answer the question from within that viewpoint, they aren't answering as a whole person; they're answering from an idea as an idea.
The question can only be answered as an abstraction within an abstract context rendering it fundamentally meaningless outside of a perversely narrow scope. In other words, its like a crossword puzzle.