What is Dave thinking when he throws his mother's ashes in the lake in A Heartbreaking Work of Staggering Genius?

This scene, which takes place in the second to last
chapter of the book, is one of the most poignant of the entire memoir.  The book opens
with the death of Eggers' parents and his tone, seemingly throughout, is one of detached
maturity, dry humor, and dilligence.  Knowing he is responsible for taking care of his
siblings, Eggers writes with the strength and courage he probably hopes to embody, and
it is clear that he is aware he only pretending to be an
adult.

When he returns to Lake Michigan to scatter his
mother's ashes, it is almost as if he is finally admitting all of the feelings he has
felt all along, but because he has been the "man in charge," he's been hiding them.  His
thoughts, a reflection of his emotions, are raw, and full of anger, bitterness, and
guilt.  Yet, amidst these, he also has a rational sense of duty and
acceptance.

His anger and bitterness (at himself, at the
situation) is evidenced by his recurring use of the f-word in a non-humorous way.  He's
surprised by how light the ashes are when he begins to throw.  He gets angry at himself
for dropping some, as if this is a reflection of his inability to do anything right. 
This subsequently speaks the message, "Why was I put in charge? 
You should have known I couldn't really handle it."  Moments of humor also sprinkle this
scene ("I'm stepping on them!  Of course I am...how fitting!  How expected,
a**hole!") to show that Eggers is attempting to avoid becoming overly
emotional or taking things too seriously.

Though the scene
is real-time was probably short lived, Eggers has so many different things running
through his mind, it seems stretched out.  He wonders if this moment is "beautiful and
noble and right, or small and disgusting."  Then, as if he realizes how introspective
he's being, he begins throwing the ashes (and now small pebbles) faster, as if to simply
get the job done and not make such a big deal out of it.

In
short, the entire experience is completely overwhelming, and as a glimpse into the mind
of someone experiencing hundreds of emotions all at once, it seems Eggers provides every
detail possible.  The reader can't help but respond with sadness, respect, and even a
little laughter.